The original Civilization had the Pyramids wonder. With just Masonry technology needed to build them, they allow you to change your government at whim, to any type of government (even if you haven't yet researched it), without an anarchy period. That basically allows democracy (a game breaker government all in itself) about 3000 BC. And then, if you ever need to kick someone's butt but the Congress overrules it, just start a revolution, declare war, and continue enjoying benefits of democracy the very next turn! The only thing you need to watch out for is to never study Communism, which disables the Pyramids effect, and luckily, it sits pretty high up in the science tree and you don't really need it.
The Civilization II Pyramids were also a game breaker as they counted as a free Granary in EVERY city you have from when you build it (right at the start of the game) right up to the end. What's that mean in real terms? Double population growth in all cities and it never expires!
Civilization IV still has the Pyramid wonder, with pretty much the same effect - it enables all government civics (Civ IV's equivalent of government types). Combined with stone in one's starting area to halve their cost, this changes them from "hellishly expensive" to "doable", and constructing them gives access to the representation civic, which both allows you to increase the population of your largest cities by around 30-50% for the time period you are in and doubles the research produced by scientists.
Some of the Unique Units in Civilization IV also have a vague Game Breaker-esque status. The Incan Quechua is available from the beginning, cheap and cost-effective against archers, making early rushes for the Incas much easier than any other civilization can pull them off. Also, the Roman Praetorian, which is an early Classical Age unit... with an unit strength more common for the Medieval Era.
Civilization II gives a number of game breaking wonders, such as Leonardo's Workshop, which upgrades all your units to more modern equivilents when the necessary technology is researched.
Civilization III had the Small Wonder "Wall Street". Its effect? Giving you 5% interest on your treasury per turn. After a few turns you had no money issues for the rest of the game, as long as you kept your treasury above zero. It was later fixed with an Obvious Rule Patch, capping the generated income from the building to 50 gold.
In Civilization I, there was supposed to be a major disadvantage to Republic and Democracy: you have to accept any peace offers. But there was a very simple counter: just don't ever meet with representatives from other civilizations. In Civilization II, they tried to eliminate this tactic by allowing a civilization to force a meeting with you any time you move a unit next to one of theirs. Since this ability was exercised most commonly after you take an enemy city, the response to this was to eliminate the defenders from several cities, then take over all of them at once.
If another civilization is getting ahead of you technologically, you can take over a city (which allows you to steal a tech), move all of your units out of the city, let them take it back, then retake it before they have time to fortify their troops. Rinse, repeat. Add in some tech-stealing diplomats, and you can eliminate several centuries' worth of technological lead in a few turns.
Civilization I had a bug that allowed each Settler to perform any one Settler action each turn (some of them were intended to take more than a dozen turns). You could also, strangely enough, build railroads in the middle of the ocean.
In Civilization II, you can produce unlimited Hides caravans and re-home to a city with high trade. With an advanced civilization, this can result in a new tech each turn, and more money than you'll know what to do with. The game designers apparently tried to prevent this by disallowing re-homing of caravans, but forgot to eliminate it from both menus.
In Civilization II, having both Railroad and Explosives completely changed warfare. If you had enough Engineers, you could build a railroad across any distance of grassland or plains in a single turn, then use that railroad to transport units across the railroad (all in the same turn). Your artillery only has one movement point? That won't stop you from using it to attack a city a dozen squares away. You can also use a chain of Transports to move a unit across any distance of water in a single turn.
Also in Civilization II: The UN was a crucial Wonder. Not because it was particularly useful (you could force other civilizations to make peace with you, but that was rather pointless since they'd then just break the treaty a few turns later), but because if another civilization gets it, having them constantly force you to make peace is a hassle.
Ironically, holding the UN makes it easier for a Democracy to wage war. It allows you to bypass the Senate half the time during negotiations, but this in and of itself isn't why you'd want to own it. Rather, the developers programmed the AI to automatically refuse to talk to you if you tried to contact them immediately after taking a city while owning the Wonder. This was intended to keep the player from forcing peace on an AI civ that had just lost territory, but making the effort yourself denied the enemy the chance to force a meeting on unit contact (see above). In effect, you're using two Obvious Rule Patches to cancel each other out.
In Civilization Revolution, the simplified mechanics provides a number of opportunities to completely dominate the AI even on the most difficult setting. And the Leonardo's Workshop wonder is so overpowered, since it upgrades all your existing units. Given the right circumstances, you can destroy any AI. First, always produce as many cheap, weak units as possible. As you're doing this, follow the path on your technology tree to discover the internal combustion engine. Time the building of Leonardo's Workshop so that it occurs right after you discover the engine and gain the ability to build a tank. All those cheap warriors you've been building since the beginning? They're now tanks. The game is over in 2 or 3 rounds max.
Civilization V gives us Bushido, the Japanese special ability, which causes units to maintain full defense and attack stats even while wounded. If this wasn't overpowered enough, it's possible to combine this with the Populism social policy, which grants a 25% damage bonus to wounded units. In other words, damaging Japanese units in these conditions actually makes them stronger. However, the effect is less useful in the Gods and Kings expansion thanks to the changes to the combat system, and is now actually considered slightly underpowered.
In Civilization III the Persians are considered by many to be the most powerful civilization in the game because they possess the Industrious and Scientific traits, which grants them bonuses to both production and research. Additionally, their unique unit, the Immortals, are the single most powerful ancient age offensive unit (it isn't until knights come along that a unit possesses more offensive power). The sheer power of the Immortals makes it easy for the Persians to conquer other civilizations during the ancient age and even well into the middle ages.
If you're going for a conquest victory in Civilization II, the Fundamentalism government type makes taking over the world really easy. Under Fundamentalism you never have any unhappy citizens, and buildings that normally increase happiness instead produce gold (and never require maintenance). You can also produce the Fanatics units, and each city can support up to 8 of them for free. The only downside of Fundamentalism is that scientific research is halved, which isn't a very big deal if you've already researched all the late-game tech or if you're rich enough to just buy technologies from other nations.
It takes a bit of fine-tuning to pull off correctly, but there exists a strategy in Civilization II called "Power Democracy" which allows a player with said government to run away with the game. Democracy has two significant bonuses: It boosts trade production on all tiles that already produce multiple trade arrows, and it allows cities with a majority of happy citizens and no unhappy ones to grow by one population point per turn for as long as the people get their Bread and Circuses. By maximizing luxuries over science and taxes and building the right infrastructure (lots of farms and roads / railroads), you'll eventually hit a positive feedback loop once your cities grow large and productive enough to support full-time specialists who produce income and science independently of the cities' own trade output. Once up and running, you won't need to worry about conquering continents; you'll be swimming in enough cash for your spies to buy them outright.
In Civilization V Bombers. The fact that they can be stacked while no other units can gives them a huge advantage. The Americans B-52's after being upgraded to Stealth Bombers are an unstoppable force.
Also in Civ V The "reduction in gold cost of items" effects, all combined, are very strong in the right hands, as each absolute value adds up with the others instead of multiplying with them. 25% for going 3 social policies into the Commerce branch, 15% for building Big Ben, and 33% for units 2 social policies into the Autocracy branch means a whopping 73% discount for units (would be 58% if the effects multiplied). A civ with a lot of gold in their coffers will be able to pump out a new unit in every city every turn for quite some time, and, with enough cash flow, can keep generating units as needed. And those bombers and stealth bombers from the previous point? Since they stack, you can buy as many of those as you can afford at once, and use them all on the very next turn. However, Autocracy does require entering the Industrial era, so it can't be done early into the game.
The Life faction's archers in Lords Of Magic have a longer firing range than any other unit in the game, including wizards. So when SauronBalkoth decides to raid your kingdom, all you have to do if you're playing the Life faction is to station tons of archers and arrow-spam him before he can get any spells off, then retreat when he's dead. As the goal of the game is to kill him, you win.
In a custom start, a water mage lord can start with the Staff of Drowning legendary artifact that can decimate enemy armies.
Lords of the Realm 2 allows you to raze a foe's crop fields to annihilate their agricultural economy. There's also a minor exploit that allows you to produce armies consisting of only one soldier (normally a minimum of 50). Taken separately, one is a relatively balanced tactic and one is pointless. Combined, you can quickly swarm over your enemy's fields and cause a serious amount of damage with only a few soldiers.
The late game in Master of Orion 2 had a pretty obscene example of this with the Phase Cloak/Time-Warp Facilitator Combo. Alone, the Phase Cloak rendered your ships invincible and invisible until you opened fire, at which point you would have to spend an entire turn doing nothing to get the cloak back up again, while the Time-Warp Facilitator effectively gave you two turns to every one of your opponents. (ie: Your Turn 1 -> Your Turn 2 -> Opponent's Turn 1) You can see the unfortunate side-effect of this a mile off, right? By using your first turn to open fire, then your second turn to recloak, you could annihilate an enemy fleet with utter impunity, without them being able to lay a single finger on you. Thankfully, this was changed in a patch.
Unfortunately it seems that all the patch did was start the attacking PC/TWF ship halfway through its two turns, so that if it attacked right away it would be caught flatfooted by the enemy's counterattack. The solution was simple - don't attack right away. Stay cloaked and let the enemy have his turn, after which you get two and can do the recloak trick as usual.
Also, the counter was quite obvious: Use a Phase Cloak on your own, but don't use a TWF. This way the enemy ships have to keep the cloak up for twice the amount of turns, which means their cloak will run out of energy first.
In addition there was a starship component called a Structural Analyzer, which doubled all damage that struck the target's bare hull. Not a Game Breaker on it's own, but there was also a component called the Achilles Targeting Unit, which caused all attacks to ignore the target's armor. Combine these with weapons capable of bypassing shields and season with some more general damage/accuracy boosting components and you had a warship who could wipe-out 10-20 ships by itself in a single round (a more 'conventionally' equipped ship takes 2-3 rounds just to destroy a single ship of similar size).
The Guardian of Orion can be defeated very early on with large numbers of MIRVed Merculite missiles. The battleship you get as a reward for this will, at this early stage, be so powerful that the other races can't touch it, allowing you to wipe the floor with them. Gyro destabilizers can also be used for a game-breakingly early Guardian kill because they do completely plain structural damage and ignore defenses.
There's also the extremely annoying ship build that combines Tractor Beam and Repulsor Beam. Thus, it can pull an enemy ship up close, fire the rest of its weapons at maximum efficiency, and push the ship back away before it can retaliate.
Ditto Stasis Field Generator. It can be used rapidly incapacitate most of the enemy fleet and then tear them to pieces one by one.
The original Master of Orion had a neat little exploit that involved putting a Stasis Field (immobilize an enemy fleet for one turn) on the same ship as a Black Hole Generator (annihilate 20-100% of an enemy fleet), a Sub Space Teleporter (go anywhere on the battle map before your opponent has a chance to move), and some bombs. When attacking your enemy at a colony, you would teleport in next to the planet and put a fleet in stasis. Next turn, turn off the Stasis Field (the SPECIAL button at the bottom of the screen) and hit the fleet you're targeting with the Black Hole Generator and any other weapons you've got on you. Normally your turn would end at this point - but you have bombs, and you're next to a planet. Do you want to bomb it? Not right now. Turn on your Stasis Field and freeze your enemy. Now you can bomb the planet, or not as the case may be; actually using the bombs is not part of the strategy. Rinse and repeat. Doesn't work in every situation, but this ship type is still useful for slightly less game-breaky variations on this general approach, and if you do get the chance it's pretty sweet taking out 32,000 battleships with one cruiser this way...
The Custom Race option can be used to achieve a Game Breaker effect: simply slap both the "subterranean" and "aquatic" bonuses on your custom race. (Never mind that it makes no biological sense whatsoever.) The result: a race that can out-breed any other, given a planet with water. And since most of the easily colonizable planets are "terran" or "aquatic" by default... And more people means more production and science. Plus the subterranean bonus gives a big boost to ground combat when defending against invaders.
The Creative racial trait borders on game-breaker, requiring only 6 of your 10 available points to buy but allowing you to research all of the technologies available, not just one out of every two to four options given. Like, say, the various technologies referenced in this section.
In version 1.2, the Plasma Cannon (specifically, the Heavy Plasma Cannon) was this. Whoever decided to make it that small apparently failed to realize that Enveloping doesn't just mean four times the shield penalty - it means four times the damage. And the Heavy mod largely negates the weapon's worse-than-usual range dissipation. It was entirely possible to lose a late-game battle solely by researching Disruptors and having all your bases "upgrade" automatically. (In later versions, they made the thing 2.5 times as big, and it was still in the top class of late-game weapons along with fully tricked-out Phasors, Disruptors, and Gauss Cannons. The difference being that those weapons needed a few levels of refinement to really get good, while the Plasma Cannon was that good as soon as it became available.)
Artillery has natural no range dissipation, and Armor Piercing mod: not only armor on the target doesn't matter at all, but its drive usually breaks before hull hit points are gone. Theoretically is negated by Heavy Armor early on. Practically, AI ships almost never has it. So good drives to keep the distance, Mass Driver, 1 more level in Force Fields or 2 for auto-firing, and never mind missing jammers or shields: small mass drivers are good point defence and heavy ones do flat 9 damage, so you can plunder missing tech from crippled and boarded ships with shields up to class V (3-4 levels over such guns).
In Shining Force, arguably, Domingo. Let's see... he flies, he levels up quickly, he gets some very powerful magic, he gets more MP than most spellcasters, and, most unusually for a spellcaster, he gets very high HP and physical defense, to the point that even enemies in the last few battles will probably only do 1 damage if they use a non-magical attack. On top of all that, as a spellcaster, he's a prime target for the AI, so you can use him to keep enemies' attention while your other characters wail away. His only major downside is that he doesn't get to equip any weapons, but since he's not likely to physically attack anyway, that hardly matters.
In the GBA remake, Domingo is even more broken; because of the change to Desoul, the token instant-kill spell being a flat low chance for every non-boss enemy to a spell that has a 100% success chance for all but bosses and certain monsters, Desoul is now a very powerful and efficient spell for killing monsters - even factoring its steep 8MP cost. And with Domingo's massive MP growth, he can happily spam Desoul without ever worrying about depleting his MP pool.
In Shining Force 2, one of your characters, named Karna, learns a buff spell called 'Boost 2'. The spell itself is not the broken part of it, all it does is increase the affected member's defense and agility, which is nice, but not game breaking. What is game breaking, however, is the large area it can cover, allowing Karna to affect the entire party, which no other buff spell does. Because it affects the entire party, Karna earns the maximum possible experience every time it's used, no matter what level you are. This allows you to power level Karna to ridiculous degrees relatively fast.
Now promote Karna to the 'Master Monk' class using one of two "Vigor Orbs" found in the game and buy her a pair of knuckles, she is now a very effective front-line fighter as well as a healer, in addition to leveling disgustingly fast. Now consider that you can recruit her and do all of this about halfway thru the game.
In Sid Meier's Alpha Centauri the supply crawler unit, which the computer never uses, gives the player a major advantage by itself. What makes it a game breaker is when you upgrade cheap crawlers and contribute them to builder wonders, because the cost of upgrading units is about 8 times cheaper than trying to pay for buildings. This makes it possible to build wonders, even the late game ones, in a single turn. Even at the highest difficulty, you civ score should skyrocket right after getting the tech for crawlers.
Similarly, you can just abuse the fact that the AI never uses Supply Crawlers, by using them as they were intended, ad nauseum. Supply Crawlers act as if you were harvesting resources from the square it's sitting on(though only one resource at a time, as opposed to the usual three). However, by planting forests, which have a chance based on your Planet rating and eco-damage to automatically spread, you can plop crawlers down in squares your cities can't reach and use them to gather minerals to make supply crawlers to gather minerals...
And this says nothing of the use of "Thermal Borehole" improvement... as a weapon. Industrial development is normally limited by Gaia's Vengeance, including rapid sea level rise. Boreholes tear the environment a new one. By using probe teams - saboteurs - in every enemy city to destroy the one building that lets them survive submersion, it's possible to eliminate EVERY OTHER CIVILIZATION IN THE GAME AT THE EXACT SAME TIME without declaring war even once, by using thermal boreholes to raise global sea levels until everyone drowns.
Note that this won't work against water bases. This makes the above-mentioned tactic utterly useless against the Nautilus Pirates in the expansion whose first base is always in the water.
Also, in Dread Lords and Dark Avatar, the Mind Control Center wonder is bugged and instead of giving its advertised effect (near-instant defection of influence-pressured planets), it gives an even better flat 100% boost to your civilization-wide Economy. Of course, you again have to be Evil to get it - basically making Evil the game-breaker alignment. Of course, in the Twilight expansion, both are fixed.
You can get these techs even if your alignment is Good or Neutral. You just can't research them, but there's nothing stopping you from trading with Evil civilizations for them. You don't get any morale penalties for using this tech, by the way.
In Jagged Alliance 2 fighting at night pretty easy if you have a squad (with the Night Ops skill) and equipment (NV Gs and break lights) for it, as enemys have a low field of view, accuracy, a have a tendency to run straight at your mercs if you attack them from a good enough distance (most likely because they are trying to get in range to counter attack)
The character creation "personality test" (which determines your starting merc's special abilities and, well, personality) lampshades this: One of the items you can pick from an army surplus store is "a pair of night vision goggles and the illustrated manual 'Daytime - an unnecessary hazard'".
In unpatched Heroes of Might and Magic V, the Necromancers were obscenely powerful because of an unfortunate synergy between the fact that their Necromancy skill allowed them to raise a sizable percentage of all living enemy creatures as skeletons, in a game that allowed you to upgrade your skeletons to skeleton archers on turn 2. Some neutral-monster-bashing later, and you have a stack of archers numbering in the thousands, growing to the tens of thousands, in a game where a single stack of the weakest units numbering 1000 is (supposed to be) a rare event. Cue one-shotting anything. Cue inevitable nerf, by restricting number of creatures raised per week and causing enemy casualties to be raised by creature level. Yes, necromancers were nerfed by allowing them to raise more powerful creatures.
Which is NOTHING compared to those fucking ghosts from Heroes of Might and Magic II. For every unit they kill, a ghost is created. Combined with the large number of wild low level creatures with low defense, building up a 1000+ army of ghosts is very easy. This makes maps where your enemies get ghosts all the more painful since if you lose a single fight against them, their armies can potentially get STRONGER even if you kill all of their other units. I'm looking at you Ghost Planet...
Demonology and the "Summon Elemental" spells in Heroes of Might and Magic IV border on this. Especially if the character with the spell or ability also gets their hands on a Demonary or Ring of the Elementals. Grandmaster Necromancy continued the usual habit of being ridiculously broken by allowing necromancers to raise vampires, which were incredibly dangerous and durable units, allowing the necromancer to create a snowball of fangs and murder as enemy armies were wiped out and added to the growing horde.
Heroes of Might and Magic IV broke from the mold by making your heroes able to participate in battle themselves. Combat heroes in particular could strike first, strike twice with every attack, take minimal damage from an attack and also be completely immune to magic. With proper spell support, you could give them incredible offensive and defensive abilities to turn them into absolute wrecking balls. Campaign heroes in particular could be carefully herded through academies and stat boosters to accumulate so much power that they could essentially solo maps inside of a month.
In Heroes of Might and Magic III, skills could make or break a hero. Mastery of Earth turned Town Portal into a spell that allowed a character to zoom all over the map, effectively defending every single town you had with one unbeatable army, while mastery of Air let you use Dimension Door and Fly to sequence break your way across the map. Logistics was a vital skill to zip around the map, and a hero that specialized in it was basically an untouchable rocket. Meanwhile, necromancy specialists could amass terrifying hordes, while a hero who specialized in Offense, Defense, or Archery could turn an army into a well-tuned murder machine.
Combination Artifacts were introduced in the final expansion pack, and just about all of them were amazing. A fully upgraded combination of necromancy artifacts let you produce an army of liches instead of skeletons and simply shoot everything that came after you. Perhaps the easiest to acquire was a set of armor forged from a bunch of low-level common magic items, which wound up casting a variety of painful debuffs on the entire army that could last most of the fight and even could bypass spell restrictions.
In the release version of Master of Magic, a single unit of Paladins could slaughter just about any enemy in the game.
To clarify further, the bug was that a unit with magic immunity was immune to all attacks from magical creatures, even non-magical attacks. It was supposed to make you immune to attacks like fire breath and stoning gaze as well as magical spells, but not claws and teeth. So an immensely powerful monster like a Great Drake simply could not harm a unit with magic immunity. The last official patch fixed this bug.
Sorcery gives Flying InvisibleAnything with spells and equipment (for heroes). Flying Invisible Warships unlike Paladins weren't nerfed in the patch. If Flight is dispelled on a wrong terrain units are killed, but Spell Lock is a Sorcery spell, so Flight users are likely to have it too, and even if not, there are 3 stacking bonuses against dispels.
Life magic combo of Morale + Income bonuses. Nodes may be taken by Guardian Spirit from Magic Spirit, but not vice versa, so Life wizards have an advantage of irreversibly robbing anyone else. This may or may not be balanced by the chance to overtake and getting the units there in the first place, depending on map size and magic setting.
Some strong enchantments, like Time Stop or Zombie Mastery. Hard to cast, but upkeep costs are modest.
Death has two spells giving Weapon Immunity, Wraithform and Lycanthropy... but worse are Evil Presence (crippling unrest) and Warp Node (it sucks mana back in, and the victim needs a lot of mana to have a good chance to disenchant it - a vicious circle).
Black Channel turns a unit into undead; it now no longer needs any upkeep except for one mana, but can't heal. Use this on trolls...who still regenerate. No more massive upkeep, but still healing benefits! (Since undead don't gain XP, you'll want to train your trolls up to elite first, but that's easy enough.)
High Elven longbowmen do full damage at any range. Two longbowman units can often clear the field of enemies entirely without magic, and then you give them alchemical assistance.
Undead Hordes in Disciples 1&2. One can get ghosts with physical immunity pretty early in the game. And then easily kill all those giants, trolls and krakens for large amounts of gold and exp, which in turn would allow to kill more dangerous enemies, such as dragons and enemy heroes. All handicaps enforced by such strategy are counterable. The most practical way to deal with such party is to bombard them with most powerful spells available, which works only slightly better due to the fact that your whole army consists of Squishy Wizards (while the survivors of normal army would be easy pickings for exp). And computer doesn't do that.
In the Mega Drive/Genesis port of Heroes of Might and Magic: King's Bounty, you're racing against the clock to defeat all the bad guys and save the king. Luckily, there is a spell called "Time Stop" that briefly freezes time. The Sorceress has many spell slots. The result of using these two together is that you can leisurely sail around picking up money (which always respawns). It also freezes all the random encounters on the map, letting you either easily avoid or only attack the weak ones for easy gold (and possible recruits). Yelling "Za Warudo" while using this exploit is stupid.
In Valkyria Chronicles the "Scout Rush" tactic (ignoring most of the enemies in favor of dashing across the map with scouts (who have by far the most movement points) and capturing the enemy base) is a mild Game Breaker from the beginning of the game. As you progress through the game you will unlock the Elite level for your scouts (even greater movement points), the "Caution" order (greatly decreased damage from enemy interception fire), the "Demolition Bost" order (lets scouts skill tanks with their rifles), and potentials including "Resist Crossfire", "Double Movement", and Alicia's "Mysterious Body" (free healing). By the time Windmill Plaza is unlocked as a skirmish, you should be able to complete it with Alicia solo in a single turn. Much earlier, the Kloden and Fouzen skirmishes can both be beaten by a team of three scouts in two turns.
Then there's Alicia herself: her accuracy and evasion stats are absurd, and the combination of Mysterious Body and Valkyria mean it can be very, very hard to kill her. If you're not worried about how long it takes and leaving the rest of your command-point-bearing officers in the base, Alicia is quite capable of completing several missions by herself.
In H-Game Sengoku Rance, you'll see a few people who will mention Omachi, Kenshin + Natori + Uruza + Aburako Dosan, and Yuzuhara Yuzumi + Mouri Teru pop up in discussions on who's the game breaking unit.
To explain this better; Omachi is absurdly broken. She's easily the strongest diviner in the game (fitting since she's the former queen of the Youkai empire) and her second Lightning ability is the equivalent of an I Win button. For just one turn of preparation (which can only be canceled by two ninja attacks), she can deal 150% of her usual damage to all enemies.
The Kenshin + Natori + Uruza + any monk with convert action is also absurdly unbalanced. Kenshin has a passive skill that lowers all enemies moves by one and boosts her stats. Natori has a spell that, given one turn of preparation, kills 30% of each enemies troops but has the downside of taking up all of her moves. Uruza also has a hit all ability that lowers all moves by one again. The Monk can then use convert action to let Natori and Uruza have multiple uses of their skills.
The Yuzuhara + Teru combination isn't as broken as the other two examples but it's still absurdly powerful. Yuzuhara has a very powerful short range attack that has a chance of annihilating the enemy unit. Teru is one of the best foot soldiers in the game thanks to the Ally Guard Plus ability (Very high odds of her unit taking an attack aimed at another unit) and Counterattack Rate Up 2 which give a massive increase to her counterattack damage. When equipped with the Dragonfly Cutter, she deals insane amounts of damage when counterattacking.
Tactics Ogre has too many to count. Archers are available early on and will be useful for the rest of the game; unlike any melee unit they can attack at ridiculous range (greatly enhanced by having high ground) and aren't subject to counterattacks, can walk through water, and do just as much damage as a melee class. Mid-game you have summon magics which can basically one shot a target. By the end of the game you just pick how you want to be broken - do you prefer to kill everything on the screen within a few turns with dragon magic, or do you prefer to exploit the Retissue spell to make a character with 10x the stats of anyone else who can wipe out the entire enemy party before they even get a chance to do a SINGLE MOVE?
The PSP Remake removes the Retissue tactic, but some characters still manage to be overpowered, such as Canopus, who is a winged human who has high movement, is unrestricted by height and can be turned into one of the strongest archers in the game provided you have a class mark for it...which is 300 Goth...when an average battle rewards you with several thousand. Canopus can also be kept as a Vartan, allowing him to use a bow and axe, but they both need to be one handed and they don't do as much damage...but give him an absolutely amazing movement range. Did I mention you receive Canopus on the 4th mission of the game? The first 3 missions are all automatically performed, you can't lose.
Knight Of Lodis had that any class could equip any weapon. Simple, right? Yeah, but then you've got a ghost that teleports to the target, floats, and uses a hammer. Or you can just use two insanely powerful spears (The second-most powerful weapon (after the four Snapdragons) in the game, the Spear of Longinus, is a spear.) and have your spear-using characters fight each other, hitting the target in the middle twice. (Does not work on Shaher, who stays in the corner.) And then there's Summon Magic...
A few more for Ko L: The four Snapdragons are on a completely different plane of existence compared to all your other weapons. You'd be very happy with a weapon of attack 60. Snapdragons: attack 75 right off that bat, and then you can improve that AND give your character stat boosts and special abilities by 'snapping' specific classes and characters. The Ninja and Swordmaster classes are both stupidly overpowered: Ninjas move three times faster than any other character, are available at the beginning of the game and have an insane evasion score. Swordmasters are the same as Ninjas, but trade a tiny bit of speed for the ability to equip any sword in the game, and are cast buffs or status spells. Including Time Stop (Stops time for the enemy team) or Teleport (teleport), which are already Game Breakers of themselves. Then you have sword techniques that cost a tiny bit of HP to use, but easily allow you to hit the damage cap if you have a Snapdragon... by the end you can beat the last boss with your main character and another (to use the Lance of Longinus to break the last boss's invulnerability field). In three turns (one turn to get there and break the field, two turns using of the time-stop special technique combo).
Odium has the Attracting Device. When you deploy it on the ground during battle, monsters become hypnotized and won't attack you, instead trying to reach the device as fast as possible, thus earning you several turns of blasting them to bits without fear of retaliation. Some monsters are immune, but most aren't (including, ridiculously enough, human enemies, making several potentially tough fights a doozy.) A fun thing to try is surrounding the device with your men to keep enemies from ever reaching it...
Milanor from Yggdra Union gets very strong by the half of the game. Enough that his attack can top any kind of enemy even though he has a disadvantage in a weapon triangle rule. And he's one of the main characters you need to use in every battle.
Most of your HERCs can mount up to four accessories. One accessory reduces an enemy's accuracy against that HERC by a flat 25%. Putting four of them on every HERC would turn the entire opposing force into Imperial Stormtroopers. The sequel changed these accessories so that smaller and lighter HERCs gained much more benefit than heavier ones.
In multiplayer mode, forces can be restricted by the number of credits spent in building them. This is fine for HERCs, which have standard equipment costs and never depreciate in value. The BioDerm pilots, however, have finite lifespans, and their value is dependent on the number of years they have left to live — not on their skills or rank. Through a bit of grinding, it's possible to enter your multiplayer battles with an elite pilot force (with 1 month before everyone's natural deaths), and more credits redirected to better HERC equipment, while still adhering to the same maximum credit restriction as someone with fresh pilots and weaker HERCs
In Star Trek: Birth Of The Federation, all factions have more or less equivalent ships with slight differences in speed and firepower. However, the game balance is way off. For example, nearly all Romulan and several Klingon ships have cloaking devices, which gives them a free turn at the start of battles, which often decide the outcome. Their ships are also invisible on the strategic screen. However, the Romulans are hampered by their extremely slow speed. Then you have the Defiant-class heavy escorts, which only the Federation has. No other faction has an equivalent class. The Defiant has superior firepower (higher than that of a Sovereign) and superior speed. The proper use of these can result in pretty one-sided battles.
The Romulan scouts could be potential game breakers as they are fast, long range, cheap and can cloak. Build enough of them—like 45 or so—and the fleet can take out a Borg Cube in their opening volley.
The first one is Ellis who just needs to get to level 25 to get her second Limit Break Regeneration. What it does is that it fully heals all your party members in the screen. Nothing much to it right? What makes her into Game Breaker status is her attack range going up to 10 in a grid where it's at 12 at most! Link her with another character and watch as you slaughter enemies left and right at anywhere thanks to her range.
Next is Plum and her second skill Maiden's Prayer which you get as soon as she joins you (she's level 40). What it does is that her skill gives the other party members SP, sacrificing her own. Pump up her SP in the first turn using the other party members, bunch them together, and let loose her Limit Break. Seeing as you've pumped up the other members by them getting 5 SP per them getting Plum's SP up to 200, they'll reach the 250 SP mark in no time. In which you can destroy enemies left and right. You'll only get an A rank at most, but considering the fact that it's a miracle to have a S Rank on bosses, this is pretty much a Godsend.
Speaking of which, Unleash All can also be a potent Game Breaker by itself seeing as party members who are down to 25% of their HP will have an obscene raise of all their stats.
Due to the game's rather primitive and sometimes crippling AI it is possible to win every single game in Hogs Of War by poisoning the enemy using a gas grenade and then using the Scout/Commando/Hero technique Hide to simply wait out the clock. Using this technique you can even defeat the otherwise ridiculously strong Legend class without sustaining hardly any damage whatsoever. Coming in at a close second on the crappy game mechanics you can exploit bandwagon comes the Spy class Pickpocket ability. Steal the bazooka or sniper rifle off an enemy Hog and they become near enough completely harmless.
SD Gundam G Generation tends to have a few. Units such as the Nu Gundam, Ex-S Gundam, and Wing Zero sometimes qualify depending on game, but always good units.
DS- Hyper mode units (mainly the Crossbone X-1, Zeta, Double Zeta, God Gundam (mainly if you get the other G Gundam side missions) as these units not only enjoy enhanced attacks, powerful finishers, but also some nice defensive options. Hyper Mode Nu Gundam comes close as its Fin Funnel barrier blocks everything as opposed to just beams. There's the Ex-S Gundam for characters who can't get a hyper mode- Good stats, nice weapons, if it gets knocked down to a low amount, its pilots' stats are boosted nicely, turning a so-so pilot into an ace. For Pilots in addition to high leveled Newtypes (who can randomly just avoid all attacks in a battle scene), Cordinators (mainly Kira and Athrun) and Artifical Newtypes have skills that push their stats pretty high.
World - While there are various good units in the game, a few stand out from the rest: The Halapas Gundam (Think former gamebreaker Phoenix gundam at around its former game breaking level.), Barbatos, an original mech with nice stats, powerful attacks. And last but not least, Dark History Turn A, which has similar stats to Barbatos, but various broken features and two inexpensive but powerful attacks. Though you don't get these until you beat the game. while paling to those three, the 00 Raiser and the 00 Quant are both very good suits with nice stats, attacks and a map wide healing move. In terms of Pilots, you have 0079 Amuro who starts with a skill that raises any white and/or Gundam type mech by up to 25 points; this covers all the best mechs. There's also Movie Setsuna whose skill list is very good (though you won't get him until pretty late in the game) and Aploda Nuroi, the game's Big Good with the highest starting stats of any pilot and some nice skills
Over World - in the face of some nerfs, many of World's best mechs are just about as good as they were in the previous game and it introduces some new ones: The Knight Gundam and the two Musha gundams, all three are close range Lighting Bruisers, though unless you used a promotion code from a Japanese magazine, it'll be a while before you get them. The Unicorn Gundam received some buffs, particularly a move for its NT-D mode that takes advantage of its canon pilot's stats/skills. The Pilot list remains similar though with an universal skill list, only signature skills and stats set apart pilots in their rankings. However with the Master Skill system, Characters such as Code Phoenix, Aploda and Code Ameras have special skills that affect more than one unit and with some very powerful effects.
In No Greater Glory, it is possible for the Union to hold both Charleston and Norfolk on the first turn, even though the Union player was clearly intended by the game designers to lose both. This is especially galling with respect to Charleston, since in real life, there was only a tiny federal garrison of 67 men at Ft. Sumter. Since the game's smallest army increment is 2,000 men, that's how many men the Union starts with in Charleston, with the same number in Norfolk. Normally, the presence of these tiny Union garrisons so inflames local opinion that it provokes spontaneous uprisings of 8,000 men in each area, overwhelming the two garrisons, even though both garrisons are made up of federal army regulars, and therefore are more experienced than the raw levies they are fighting. There's not much you can do to help either garrison as the Union player, however, because on the first turn, you only have enough sealift capability to move 2,000 men, and cannot send any reinforcements into Charleston anyway, because of the blockade. What you can do, however, is pull those 2,000 men out of Charleston, and send them to Norfolk. Without any garrison in Charleston, there won't be any Union presence to provoke an uprising, and the area will remain under Union control! Meanwhile, 4,000 regulars in Norfolk have chance of holding the place. Then, if you can declare martial law in the tidewater region, where Charleston is located, during the next civil affairs phase, you have a chance of holding it, although there certainly will be riots, because, weirdly enough, the game doesn't actually require you to have any troops in a place to impose martial law! And then, of course, in the second strategic movement phase, you can start sending in reinforcements to both areas with newly built transport ships. As such, you can start the game with two of the Confederacy's ports under your control, one of which is one of the Confederacy's major cities, which you need to win, and the other of which is immediately adjacent to Richmond. This fairly simple trick starts the Union off with a huge advantage.
Advance Wars has a nasty strategy akin to the memetic Zerg Rush. On any deployment map you can just keep producing Mech units and swarming your opponent with them. Mechs are strong enough to take on units like tanks and artillery, can cross mountains and rivers, and can capture bases. Best of all, they cost only 3000G to produce each turn. They die fast, but since there are so many of them, your opponent will be overrun and pushed back, allowing you to easily capture their HQ.
The sequel introduces us to Lash, one of the Black Hole Commanding Officers. Her ability increases a units offense by 10% for every star of defense a terrain provides it. In other words, parking units on high-defense terrain not only makes them hard to destroy, but they now also pack a nasty punch. Rockets on her HQ annihilate anything that comes near, mechs on mountains can take on medium tanks, and battleships on reefs rape everything in sight. Her CO power actually ''doubles the defense level of all her units as well, which happens to also double their attack bonus, and her CO power fills up rather quickly. She also has no weakness. Even after being Nerfed for Dual Strike, she is still one of the best characters available.
Age of Wonders has some generally nasty units, but the human air galley is generally considered especially so. It has flying, which means that units without ranged attacks or flying are unable to engage it. It's also a flying ballista, which are usually given special consideration by AI when on the ground. It can also transport up to seven other units, giving otherwise slow but powerful units a boost and dropping units into places otherwise inaccessible. It also has decent stats and, being a machine, is immune to such effects as fear.