In any game where building is based on adjacency to other structures rather than builders, there's the possibility of "base walking;" constructing a line of the lowest-cost structure that still has build adjacency towards somewhere you want to go. Whether this is simply another tactic or a game breaker depends on how into Tactical Rock-Paper-Scissors the game itself is; if base defences hose ground units, it can be used to very rapidly gain control of map areas with static defences that have no real business being where they end up. Dune II was the ur-example, since even cheap-as-free concrete tiles had build adjacency.
This was even turned in a sort-of requirement in an early Command and Conquer game, because otherwise the enemy would send countless reinforcements by sea so unless they were blocked from accessing the beaches the player couldn't actually focus on the mission.
Plus Lord Kane's Smooth Operator ability will freeze any agent he targets with it until he gets into range to actually use the ability. Even your basic worker can knock out a Super Agent if Kane has a long way to run.
If you manage to build an Über-trap and put agents into it, you've got an instant and limitless cash cow, as the game awards monetary bonuses to trap combos. With several agents in the trap, you will never need to worry about funds again.
In the game Star Control 3, the race of Doogs starts out an enemy, but can be converted into friends early in the game. Their ships are maneuverable, relatively fast, have a short-range cannon which can auto-fire on nearby ships, AND can quickly regenerate damage. No other enemy ship in the game comes close to challanging a Doog ship, making basically every battle after you get a Doog ship a matter of selecting the Doog ship and blowing up the enemy ship.
Star Control 2 will try to keep you in Earth's solar system until you gather the Plot Coupons with hordes of tough Slylandro probes. Fortunately, there's a Spathi ship lying nearby that's just perfect for killing them and gathering resources by the thousand before the game story hardly begins.
That same Spathi ship walks all over the main antagonists (the Kohr-Ah) when you run across them. Probably lucky for the rest of the galaxy that the Spathi are cowards.
Makes you shudder at the possibility of the Black Spathi Squadron.
There is one ship that is banned from serious netplay, and that is the Thraddash Torch. While it does have a counter, it can destroy most of the ships in the game without much risk, due to the fact that it's a Fragile Speedster (in fact, the fastest ship in the game) and has the range to pick just about anything without giving the opportunity for return fire. It also makes the game incredibly tedious when someone does this, because said long-range weapon, while completely safe to use, does 1 damage and has a low rate of fire.
This ship is principally broken against the AI, which is often not smart enough to avoid running into its secondary weapon.
The Balance in Command & Conquer, especially in the early installments, tended to be a little off. It's gotten better in recent times, but Game Breakers still pop up.
Right from the strat, the first game had GDI grenaders. Capable of superior AoE damage against infantry than minigunners and decent damage against structures and vehicles, the Grenader could also be built both early and cheap, allowing a GDI player to just spam them to victory. Though the minigunners were cheaper, the grenader could hit all of them with a single grenade, allowing him to wipe out many times his own number.
Fortunately, they were toned down in RA1, where the Soviet version did slightly reduced damage and, to prevent spamming them, exploded upon death, which could set off groups of grenaders like firecrackers.
Early Command & Conquer games were infamous for having tank spamming being virtually uncounterable strategy [[except by an opposing tank spam]], giving them game ending status. In fact, it wasn't until RA3 that this strategy was finally wrestled under control, by making anti-tank infantry much more viable.
In original C&C, the AI would not try to destroy sand bags that blocked its movement. (It would shoot through walls and fences that blocked line of fire, destroying them in the process, but sand bags didn't block LoF, so they would never be destroyed unless they were close enough to another target to be hit by stray shots). This meant that if you could build a sand bag wall round you base, far enough out from any target, then the enemy would be unable to get in. Then you could extend the wall into the enemy base, and wall off all their unit factories. Or build Advanced Guard Towers / Obelisks of Light inside their base. Or build a barracks in their base (particularly effective if you simultaneously build an Engineer, timed to complete just after the barracks completes, for instant All Your Base Are Belong to Us).
Tiberian Sun, before the Firestorm expansion pack had the bugged Nod Artillery unit, which could always hit it's target, even if the shell itself didn't hit anything.
Also from Sun, there were the GDI disc throwers. A combination of good damage against everything (unlike the Nod rocket infantry, which had a crippling weakness against infantry), low production cost, quick production time, and a Good Bad Bug that allowed them to throw discs across the freaking map basically meant building an army of disc throwers was a perfectly viable strategy for most levels.
Red Alert 2 had the extremely broken French subfaction, which came with the Grand Cannon, a huge defensive artillery piece which could destroy any enemy in the game with ease. As air units, it's only counter, were rare and underpowered, a French player could simply slowly creep his base towards the enemies under the constant protection of his cannons, until he was shelling the enemy base.
Ditto the British subfaction, which has Snipers, who could insta kill any infantry unit in the game.
Yuri, the faction added in the RA2 expansion, was an game breaker faction. It's mind control tech was an instant game breaker in and of itself, making any notion of cost-effectiveness totally null, especially as he could recycle these units for cash. Their resource gathering mechanic allowed them expand lightning fast and claim new ore fields that the enemy could never touch, and their flying unit could shut down whole bases with ease in a game where flying units and the defenses against them were both seriously underpowered, and even steal from the enemy directly. Yet, even with all these advantages, Yuri was no worse in direct combat than his foes, with good tanks, rock solid defenses and excellent basic infantry.
Rare case of Game Breakerlampshaded in-game: Demolition Truck + Iron Curtain combo. Let's recount: Demolition Truck is a suicide unit carrying a mini-nuke that explodes if the unit is killed or if the unit is commanded to attack something. Iron Curtain is a minor superweapon that makes the targeted nine units invincible and immune to mind-control for a short time. Do the math. At least the original Red Alert had the courtesy of causing iron curtain to wear off very quickly from demo trucks... Lampshaded in Yuri's Revenge in the Soviet campaign where one mission requires you to either use this tactic or spam a metric fuckton of V3 Launchers to crack the enemy base.
Somewhat related to this, and if you play against the dumb AI, it's possible to blockade a base using Desolators. Doing the deploy command causes them to irradiate an area equal to a nuke until they are given the deploy command again. Most infantry simply melt before they have a chance to attack the Desolators while basic tanks will slowly disintegrate. In fact, in one of the Yuri's Revenge campaign maps, you could effectively blockade Yuri's base with two at a choke point.
The Mammoth Tank in Command & Conquer 3: Tiberium Wars is a beast. Easily the toughest unit in the game, with upgradeable weapons that let it kill infantry in a single hit, more firepower than any other armored unit in the game, and respectable anti-air capability to boot. Massed together, they're able to go toe-to-toe with equivalent numbers of Nod Avatars and win, and outlast Scrin Planetary Assault Carriers. Nothing short of a direct hit with a superweapon can really stop the billowing clouds of dust that herald a Mammoth rush.
Later patches Nerfed the Mammoth, but its early-game counterpart, the Predator, which had the same weapon upgrade, proved effective against all but air units.
A Mammoth cannot stand against a Planetary Assault Carrier. Planetary Assault Carriers have incredible range, kill anything very quickly, and perhaps best of all are aircraft and thus both unaffected by terrain and out of reach of the most dangerous units in the game. They specialize in anti-armor attacks so vehicles can't beat them, and infantry are sadly too slow to reach them before being vaporized. The only credible anti-air air unit in the game was not a reliable counter as they had the unfortunate tendency to blow all of their wingmates up. Despite the multiplayer imbalance issues this created for a while, it really did feel pretty good destroying everything in sight with gigantic alien airborne aircraft carriers.
Mammoth Tanks were on the same tier as the Scrin Tripod and Nod Avatar, but would be superior to either because of their lower build cost, Anti-air capabilities and immunity from being demolished by a Commando. Mammoth Tanks also one-upped the Avatar due to it being effective against all unit types, whereas the Avatar needed costly upgrades to be effective. It also couldn't be stolen by the enemy.
The expansion introduces Mechapedes, fast-moving units that can be equipped with a variety of weapon modules. With enough micromanagement, just a few can wipe out an enemy force without exposing their weak points to serious damage.
The Black Hand gains the Purifier Warmech, which was suppose to be a primitive version of the Avatar. In theory it's a costlier Avatar with a built-in flame thrower in lieu of the upgrading ability. While it resolves the Avatar's initial problem of not being able to deal with infantry, it also made the Purifier completely immune to it's hard counter: commandos. On top of that, the Black Hand has the Purifying Flame upgrade, which allowed all their flame-based weapons to deal extra damage to vehicles (normally fire damage deal almost negligible damage). Because the flame thrower is programmed to fire without the unit needing to target, a group of 6 Purifiers can literally walk right through the opponent's base, torching anything short of flying aircrafts without so much as raising their arm-mounted lasers.
In Command And Conquer Red Alert 3, the Yuriko unit from the Empire of the Rising Sun can be considered one. She can destroy buildings in seconds, can attack both land and air units, and has a special attack that kills all infantry within a certain radius around her.
In the Empire of the Rising Sun, the Shogun Executioner. Lampshaded by the Soviet Commander accused you of cheating and let the Game Breaker unit win the mission for you, instead of any real strategy.
Red Alert 3: Uprising is just packed to the gills with gamebreaking units, suspiciously almost all of them Allied units. There's the Harbinger gunship, with insanely heavy armor, and dual weapons capable of catastrophic damage against armor, or nearly insta-kill guns against infantry. There's the Pacifier mobile cannon which is AMPHIBIOUS, and when it sets up, produces an extreme range cannon as powerful as the Cruiser in RA1. The Future Tank X-1 is pretty much super effective against everything, and weak only against the most powerful armored units. The Cryo Legionnaire freezes up ALL units within a rather large radius in front of it, and these frozen units can be broken into a bunch of pieces by the weakest units in the game. The only non-Allied unit that comes closer to gamebreaking is the Japanese Giga Fortress, but even that is hardly as powerful as the Allied units.
Suffice it to say there's a reason why Uprising doesn't include any sort of multiplayer.
In Generals each faction has one. Hilariously enough, Zero Hour actually gives you the option of picking a subfaction that upgrades these tactics:
The Chinese overlord tank which is the obligatory Mammoth Expy who's counters are supposed to be aircraft and rocket-wielding infantry, both of which are negated by sticking a gattling gun on top of it which also lets it detect stealth, It get's turned Up to Eleven with the tank general's Emperor Overlord, who comes with the self healing propaganda tower by standard issue and can build a gattling cannon as well.
The GLA terrorist on a bike, Terrorists can OHKO a tank, their weakness is supposed to be that theyr're not much faster than regular infantry. Putting them on a bike lets them jump cliffs, go faster and generally make your life a pain. It gets turned Up to Eleven by the Bomb general, who can build them with the terrorist on top by standard, make them cheaper, and more powerful.
The US Aurora Bomber, which is literally invincible until it drops it's bomb, the bomber can literally be used in groups to completely eliminate anti air before blowing the entire base to kingdom come. Gets turned Up to Eleven with the Superweapon general's Alpha auoroa, which is 20% cheaper and comes with a more powerful fuel air bomb.
None of these are truly game breakers... The Overlord and Emperor suffer from being so outrageously slow that their ability to run over other tanks is almost worthless, and makes them susceptible to armies of cheaper tanks and anything that can shut them down (saboteurs, microwave tanks, ECM, Auroras). Terrorists are as easily gunned-down as any other infantry by snipers, miniguns, or quad cannons. Auroras and even Aurora Alphas are very good, but are still expensive and have a slow build time, and only do limited damage before becoming vulnerable - well-placed base defences are practically guaranteed to turn them into extremely expensive missiles (especially the GLA SAM sites, which will be fully repaired in the time an Aurora takes to refuel, and the Chinese gatling cannons, which ignore the anti-missile countermeasure upgrade).
In Pikmin 2, you get purple Pikmin. They can kill around 90% of the enemies in a few throws, making most bosses ridiculously easy (in fact, this game's Prima guide just says to throw purple Pikmin on said bosses). Their weakness? They're slow. Yellow Pikmin in the first game may also count due to their ability to carry bomb rocks.
Not only that but the purple Pikmin can home in on enemies which means you don't have to be that accurate when throwing them which can work wonders on groups of enemies. Even if they don't land on an enemy or boss they can still stun them when they land on the ground. (Bosses require more pikmin for this to happen.) They can also instantly disable geysers when they land on them.
The first Battalion Wars has Anti-Air Vets, which for being multi-hit along with causing knockback to any land unit except the Battlestation causes high damage with each hit. Want to know how bad this gets? A lone one can solo the two Battlestations in the final mission. That isn't even anything close to within their job description and they manage it anyway, along with cutting down all of the infantry along the way including the Acid Gas Vets—in fact, Flame/Acid Gas Vets are supposed to do this job. Assault Vets in the first game, as well as Bazooka Vets in the sequel, can't even compare to this.
However, let's not let either off the hook, partly because both, like the Anti-Air Vets, are infantry who can pull evasive maneuvers against enemy units, which isn't a bad thing considering the games generally center around infantry, but also because both get overboard with their power:
Assault Vets in the first game deal Heavy Tank armor piercing damage with each shot. Of course, this also means that a manually controlled Assault Vet can turn destroying heavy units (except the Battlestation, which is mercifully immune to the bullets) into a game of balance-the-meter to keep shooting rapidly without suffering the overheat. Something is wrong when a lone unit can kill the local Demonic Spiders effortlessly. And something like this that can happen sums it up:
Xylvania infantry: * sits in position ready for enemy attack*
''(Two seconds later, the Xylvania infantry are all dead, with a lone Frontier Assault Vet standing in the middle of the dead bodies and the medpacks.)
Frontier Vet: Never knew what hit 'em!
Bazooka Vets in the sequel have been toned up from the shoddy damage that a lone one deals to Light Tanks. However, problems arise because against infantry, they are Mighty Glaciers, with good potential for One Hit Kills. In addition, a lone manually controlled Bazooka Vet can solo an already nicely guarded Battlestation or 2 Battleships. A lone Mortar vet can solo a Battlestation more easily, but they just run into trouble with evasive infantry. The Lone Bazooka doesn't.
Star Trek Armada 2 gives the Borg a huge game-breaker: The fusion cube, in which 8 regular cubes are merged into one giant cube which has an insane amount of firepower, and can pretty much wipe the map on its own. But that's not the best part! The Borg also have the tactical cube, which is basically a regular cube on steroids with armor plating and super-charged weapons, and 8 of those can be merged into the unholy Tactical Fusion Cube! If you build a Tactical Fusion Cube, you're pretty much guaranteed to win that scenario. And just to put icing on the cake, a FC/TFC is not especially expensive to build.
It should also be noted that virtually every Consortium unit aside from basic infantry and the Vengeance frigate possessed shielding of some sort, which, by Star Wars canon is utterly ridiculous. And to make up for the lack of shielding (and make matters worse), Vengeance frigates packed mass driver weapons which pierced through the shields of capital starships and wrecked fighters and bombers that tried to get close. Vengeance frigates were capable of tearing through most starships like they were tissue paper. And this was not even the Consortium's most powerful warship.
To win a game of Galactic Conflict the Consortium only needs to fight using two standard units, in space the Vengeance Frigate whith the rapid fire, highly damaging and shield ignoringmass drivers. They do have more weapons, but they mostly serve as other targets to distract attention away from the mass drivers and engines. The other unit they need is for land battles, but is just as bad, the Canderous Assault Tank is also packing mass drivers, it may be their only weapon but they cannot be disabled until the unit is destroyed. They still ignore shields which makes them devastating against the handful of shielded vehicles and structures, and are themselves shielded, a single tank on it's own would lose in a fight against an Imperial AT-AT, leaving the AT-AT crippled...But there are three tanks per unit, compared to the single AT-AT.
The idea for "balancing" was the Consortium was the comparatively high prices their units had, but the existence of the Palace at a planet vastly increased credit revenue far more efficiently than Imperial or Alliance mines and only took up one building slot per planet. It is flagrantly obvious that Petroglyph never bothered to balance this faction.
Worse at launch, when the only space unit the Consortium needed was their fighters, which due to an oversight was so OP it could take out any other units in swarms, which it was totally possible to have since they were cheap. Whats worse is Petroglyph knew about the tactic but still took months to patch it.
Another thing the Rebels have going is T4-B tanks. They can switch between lasers that melt ATATs, and shield-penetrating missiles. They have absolutely no viable Zann Consortium counter except Rancors and hero units (Zann infantry are supposed to be able to take them, especially since they can't be run over, but the missiles do too much damage for the infantry to be cost effective). T4-Bs can take out turrets and even shielded structures from outside turret range. The result of this and the above Zann balancing issues is that the Empire gets positively destroyed by the Consortium and to a lesser extent the Rebels, the Rebels can only stop the Consortium later on, and the Consortium ends up powerless if they don't stop the Rebels in time.
Tzar: Burden of the Crown had a faction where you could spend money to create "relics" that could increase your units stats. Game broken when you could play with unlimited resources and have units carrying relics that increased stats by 10 times.
Supreme Commander: Play Seraphim. Get the rapid restoration upgrade on your ACU. Then march it directly into the enemy base. Only experimentals can stop a RRF'd ACU, and you can get RRF by the beginning of T2.
Supreme Commander has always had some, erm, issues with game balance, most usually favouring the Aeon.
The release version included balance issues that had been identified in the beta but not actually addressed, including the Cybran Mantis being ludicrously overpowered in the early game and the UEF Broadsword gunship likewise in the late game; both were nerfed repeatedly.
This only really let the ridiculousness of the Aeon become apparent, though; typically having the best or at worst the second best of every unit type. Aeon were all but impossible to beat on water maps, trading the useless deck gun of other faction subs for a second torpedo attack and having a basic T1 tank that was amphibious; they also had a missile defence which couldn't be overwhelmed at all, a T3 artillery which was pinpoint accurate and fired twice, and the Harbinger, a Siegebot which had the best cost versus damage of any unit in the entire game. The Aeon superiority on maps with water was at one point so severe that all water maps were removed from competitive play.
The ACU destruct nuke used to be a completely normal nuke; it was nerfed because every game was ending in draw-by-combombing.
AA weapons used to have no priority system of any kind, leading to a common tactic of building a gigantic number of cheap-as-free Air Scouts to support a Strategic Bomber attack; this effectively made air defence a waste of time.
An attempt to make the UEF T3 mobile sonar into something actually useful accidentally turned it into a ridiculously overpowered motor torpedo boat, requiring another patch to stop the ridiculous sight of flotillas of things which were technically buildings chasing battleships around.
Many people consider the King Tiger in Company of Heroes to be this. It's a one time use unit that drains 2000 manpower over the next few minutes after being fielded. It's also slow. However, it has tons of health, is heavily armored, can be called in at a moments notice, does not require you to 'pay' to field it, and and damage done to it can be repaired. It is essentially a supertank that you can call in for 'free' (you don't need to save up for it) and can be added to your existing army to give you a massive edge over the other player. The other player will need to field lots of AT Weapons to take it down, but these are costly and take time to make, and ontop of that, he still has to deal with your existing army while trying to set up and defend these AT Weapons.
The Western Allied counter to that would be the American Pershing, which is close but not quite up to the King Tiger's power. But unlike the once-per-game-if-you-loose-it-you're-screwed KT, you can literally SPAM these things ath the Germans from here to doomsday as long as you have the funds avalible. And suddenly it's the GERMANS who are having their tanks chewed up and spit out like paper. ALL OVER THE MAP. And as long as you keep some engineers nearby to do the obligatory repairs, these things gain experience easily, which makes them only more dangerous. Suffice it to say by the end, the German elite armor will struggle to scratch these things.
While the King Tiger can be a tough nut to crack, it by no means "breaks" the game. It's rather slow, and a single sticky bomb essentially makes it immobile, or at least slow enough to be taken out by pretty much anything with good armor penetration and/or long range. The turret is very slow, so the KT can be kited by anything that hasn't got its engine blown or tracks damaged. KT is excellent at soaking damage, so the American or British players shouldn't focus on it unless it's alone, but doesn't exactly destroy everything in sight within seconds. Considering the things that have been in the game (and have been removed or tweaked since then), KT seems like pretty much any "ultimate" power of any company\doctrine of any faction, and is by no means unstoppable.
If anything, the Commonwealth's artillery is a game-breaker. Once the Artillery doctrine abilities are acquired, every British artillery piece on the map can shell the enemy's main base simultaneously, reducing all that work to rubble, and forcing the enemy to slowly rebuild his entire production capacity. Setting up three or four 25-pounder artillery pieces takes a bit of resources, but then again the British have amazingly powerful defensive emplacements to boot, and can just sit back and gather resources for a while. Even a King Tiger can't stand up to a couple of British AT emplacements - they're some of the most powerful AT weapons in the whole game.
The first Age of Empires game had the Horse Archers. They can move fast, do a ton of damage, and have a range longer than anything except upgraded Priests and certain siege weapons. The only downside is that they're expensive to make and can't take much damage...at least, not until you upgrade them to Heavy Horse Archers.
Fridge Brilliance and Truth in Television. For the time period, horse archers were devastating, capable of swooping in, spraying the enemy with arrows, and then beating a quick retreat before the enemy could marshal its forces. Hell, even today that'd be a viable strategy against anything less than a fleet of attack helicopters.
The series also has elephant-mounted warriors. The Elephant Archer in the original has as many hitpoints as buildingsnote (though it has as many hitpoints as buildings, in the first game buildings take a fifth of the damage that units do), and the Persians' War Elephant of the second game is probably the strongest unique unit (good for the Persians, since they're otherwise kind of a limited civilization in-game).
In the third game, Grenadiers. They are effective against both infantry and buildings, and do splash damage. They're produced at the Siege Workshop, but they're infantry, which means they're cheaper, faster, and don't have to deploy in order to attack. They're vulnerable to cavalry, but Grenadiers can stand up to massed infantry, where an equivalent unit of artillery will not. This is particularly nasty when fighting the Aztecs, who have no cavalry aside from any native units they've acquired.
While not breaking StarCraft's famously balanced multiplayer, Protoss Carriers with an Arbiter in tow are overpowered in single player scenarios against Zerg. The Arbiter generates an invisibility field, and while being visible itself the computer doesn't bother to go after it if it's stays in the background. As soon as you can build this air fleet, you'll be able to kill AI Zerg with impunity.
And in Starcraft II, anti-Zerg missions are easy with a large amount of Reapers, who are cheap, attack and move very fast, and do stupid-huge damage to buildings. They can also jump cliffs, meaning they can strike pretty much anywhere, anytime they feel like it.
While were on the subject of Starcraft II, might as well mention a few of the mercenary groups you can hire. The Siege Breakers, the elite version of the Siege Tanks, are just...not fair to the enemy. Have all the upgrades possible and these beasts can dish out 150+ damage on single-targets (in addition to having an extremely powerful splash-damage).
The triple Viking mercenaries aren't too shabby either. They are, hands down, the best anti-air units in the game, and the price to buy this group in-game is an absolute steal for what they can accomplish.
In Starcraft II, Protoss players in 1v1 Bronze league are notorious for using "Cannon Rushes", building pylons and Photon Cannons close the enemy's mineral line as soon as possible. What makes this a gamebreaker is that your oppponent will not have anything other than workers to stop the cannon if the rush is fast enough, and the cannon will kill a worker in 2 shots, while the opponent is probably throwing down 1-2 more cannons. The Cannon Rush isn't such a big deal at higher levels of play due to everyone being aware of it and its considerable shortcomings (if the cannon rush fails you have no tech, military or economy to speak of; meanwhile, the cannon itself can be overwhelmed with early t1 units or surrounded by workers if it isn't built fast enough, especially if the opponent is scouting his base perimeter and watching for "cheese" tactics), but newcomers will be overwhelmed and maybe even turned off by this seemingly unstoppable tactic.
Bloodlust in Warcraft II is a spell which triples damage and is cast by the Orcs' primary attacking unit which is already somewhat of a Lightning Bruiser. The Humans' equivalent get extremely inefficient healing and a spell that only affects two, rarely-used, undead units (Death Knights and their Skeleton minions). Needless to say, Orcs have a huge advantage on land maps.
Additionally, the "Mage-Bomb" tactic is one strategy that can be game-breaking in the single-player campaign; If you're human, it involves making a Mage invisible and then sending that invisible Mage who can cast Blizzard into the enemy's gold supply lines and then raining a deadly ice storm upon the line of workers harvesting gold. It is devastating, if not decisive against the enemy's army production. Playing as Orc, the same strategy is done with Death Knights, and an Invulnerability & Haste spell cast upon the caster.
In Warcraft III, the Orc Blademaster hero. He's among the fastest heroes, also among the toughest with his high armor score and agility, and with Critical Strike acting over his item-boosted melee damage, is easily the highest direct damage dealer hero. But the real kicker is his Wind Walk - a spell that not only allows him to move even faster and, at higher levels, stay invisible most of the time, but allows him to make rapid escapes whenever threatened by letting him move through other units and hence escape traps. Needless to say, he's one of the best hero rushers, excellent at hero killing, very hard to kill himself if properly used and generally a pain in the neck for any opponent, with no clear weaknesses.
Always having an item on-hand like the Gem of Trueseeing to be able to see the Blademaster when he windwalks and to reveal the fake clone images helps immensely.
There's also the Human Archmage to consider, having a level 6 ultimate that allows the Archmage to mass-teleport himself and a group of your units to the location of a unit you own. Kinda like the mage example mentioned above for Warcraft II. A clever Human player can sneak a unit (especially with the Sorceress' invisibility spell) into the back of an enemy base and use the Arch Mage's mass-teleport to instantly get his army to where the workers are at and wreak havoc.
In the bonus orc campaign of Frozen Throne the tomes of Intelligence, Agility, and Strength that you can buy from certain vendor stores are this. If one is patient enough to grind gold to buy the tomes to continuously buff up the heroes they control (These being Rexxar and Rokhan, and later adding Chen and Cairne to the group), you'd end up with a group of overpowered heroes that would absolutely annihilate anything in their path.
Frozen Throne also gives the player the ability to use a pseudo-5th-race at certain points in the campaign; the Naga. Because this race is only used in the campaign, Blizzard chose not to bother attempting to balance it's units in comparison to the four main races. This is extremely apparent with the Naga's flyer unit, the Coautl. The Coautl are almost an exact carbon-copy of an Orc Wyvern, but Wyverns take four food to build while the Naga's Couatl only takes TWO. The 5th Blood Elf mission when you can just mass about 50 of these beasts to steamroll the Fel Orc bases is just...not fair.
Against the AI, the Necromancer/Meat Wagon/Obsidian Statue combo is nearly unstoppable. Necromancers to swarm the enemy with multiple skeletons, meat wagons to create corpses when the enemy isn't dying fast enough, and obsidian statues to refill life and/or mana to multiple units at the same time.
The Dark Ranger has hilariously overpowered abilities, chief among them the ability to prevent multple units from casting, creating tough melee units whenever an enemy dies, and a short-cooldown, low-mana, nearly-unblockable, permanent, Charm Person. The only ability that comes close to Charm is the Alchemist's Transmute, which instakills an enemy and gives you gold for it.
In a rare example of the game (and the Strategy Guide) recommending a Game Breaker, Reign of Chaos had the Goblin Zeppelin / Sapper combo. Load the autocasting Ao E explosion sappers eight at a time in the zeppelins, and watch them destroy a base with the enemy able to do very little about it. This was made impossible in the expansion, and carried over to the original if installed (yet the prompt to load them up still appears).
Star Wars: Rebellion is full of these. The first is the Alliance Escort Carrier. Seems unassuming, until you realize X-wings are arguably the best starfighter in the game, until the TIE Defender comes along. You can easily take down a Star Destroyer. The second is the Death Star, but only as a defense platform. Put one around Coruscant with plenty of starfighter protection and a shield, with plenty of troops to protect the shield, and hilarity ensues.
Lashes in Dwarf Fortress have an incredibly small contact area, travel very very fast, and do blunt damage. This leads to people's brains being knocked out of their skulls - it's easily possible to one-shot a dragon doing this.
Maxed out strength and Throwing Skill results in decapitating Bronze Colossi... with empty saddlebags.
In the first two installments of the Warlords Battlecry series, a character who was created for the campaign mode could not take part in skirmishes (and receive experience for it) until they finished said mode and exported their user data. In the third installment, these two modes were integrated (meaning skirmish matches could be played independently using the same campaign hero), letting a savvy player set up matches against ridiculously-hard AI opponents who had little to no resources, then reap hundreds of XP points in a short time. By combining this with a few upgrade points pumped into the the Knight Lord skill for the Knight class, a player could conceivably start a campaign mode tens of levels higher than the initial difficulty, and (by spending all starting XP on Knights) have a posse of bodyguards who will ravage anything and everything in their way. Beating the campaign then becomes trivial; it's possible to use a Knight Hero with a handful of Knights to clear out an entire map using this method (even on hard difficulties).
In early versions of Brütal Legend, an Ironheade player who spammed fire barons was virtually unbeatable. Fire baron spamming was so overpowered that it nearly killed the game's online multiplayer community. Eventually Double Fine released a patch that gave fire barons a much-needed Nerf.
In Outpost 2, Eden's Thor's Hammer has the longest range, deals the most non-explosive (i.e., Starflares and Supernovas) damage, and ignores line of sight requirements. Due to the AI's lack of finding alternate routes, a couple of these set along the path the AI will take with some obstacles in the way to slow them down (or better yet, on a cliff) will render your base immune to direct attack.
The Noble Werewolf from the final Majesty 2 expansion is easily the most powerful hero ever seen in a Majesty game. Nobody else even comes close. By the time an average hero would get around 1k HP the Werewolf has 4-5 times as much, making them extremely difficult to kill. They also get ridiculous health regeneration and a large array of conjuration skills and powerful melee attacks that allow them to simply wade into a crowd of enemies and wreck total havoc. What really makes them devastating, though, is how easy they are to create compared to other top tier units- you only need a level 2 palace and a decent amount of gold to start recruiting them. The standard Human faction really has nothing that can compare.
This is somewhat balanced out by the fact that the Monster faction which the Noble Werewolf is a part of has no basic healing units, which cripples them compared to Human armies early on.
Achron has a major problem with air units in general. Air units are supposed to be balanced by their high cost and frailty. However, when massed, air is much more powerful then ground units can possibly be, because air units can stack up in the sky, creating a rolling wall of death that will focus tons of damage on anything in its way. By contrast, ground units are limited by the amount of space on the ground, so you can't have many focus on one point. Also, air units are several times as fast as ground units and don't have to worry about terrain obstacles, so a single death ball and threaten the entire map and fly circles around their supposed counters. If they are outmatched, they can quickly retreat to heal or attack somewhere else.
In the single-player campaign of Sacrifice Sirocco the Dragon can be obtained at James' 2nd level. She is the Hero Unit of the dragon, Persephone's ultimate creature, and will one-shot wizards, buildings, creatures, and practically everything for the next 6-7 levels. She also flies and, to add insult to injury, can resurrect your other creatures. On the flip side, she will abandon you if you don't play for James, Stratos or Persephone. And, if you play for Stratos, once he backstabs Persephone.
Hood the Rabbit in the flash game Paladog. By virtue of being an archer, Hood is capable of hitting enemies before they even come into range, which is an enormous advantage as this means they can completely avoid getting hit. It's intended to be balanced out by a slow rate of attack, so that the enemies that survive a hit can get in range and take out Hood, who stat-wise is a Glass Cannon...but then you realize that Hood is very cheap to summon (as in, second cheapest unit in the game) and that you can just summon nothing but Hood the Rabbit. A slow rate of attack means nothing when the enemy is facing an enormous Zerg Rush wall of these attacks.
Total Annihilation: Kingdoms has the entire Zhon race as a gamebreaker, as they can create units anywhere without the need for buildings. The ability to conjure up an army at a moment's notice in almost any point of the map, not to mention that of relocating a base simply by walking most of it away, makes the Zhon much more versatile and efficient than the other races. As for specific units, the Zhon lightning tower is cheap, quick to build, powerful and long-range, making it ideal for both defending and attacking by fortification-walk.
Lobsters are surprisingly useful. They've got nearly unrivaled armor, the ability to swim, regeneration, and exceptionally powerful claws. Mixing a lobster with a large creature (hopefully with a powerful head-based attack like a crocodile) leads to a creature that is not only physically very powerful, but also just happens to have an array of surprisingly useful special abilities.
The archerfish has a range attack that does a lot of splash damage. Just combine something big with one and watch them destroy herds of creatures. The attack also can be upgraded to do piercing damage, which makes it even more of a Game Breaker. The only downside is it's one of the few attacks in the game that can damage ally units.
Antiair towers pretty much make all flying units useless.
The Terra Cotta Army, one of the earlier wonders, produces a free infantry unit every 30 seconds, with an additional half-second added for each unit produced. Given how long games can last, this will translate into a very large army.
In late game, the Statue of Liberty. It makes all your land and air unit upgrades free, AND reduces attrition.