In Shogun, Legendary Geisha. They're nearly invincible and can wipe out entire clans if used right.
The Hojo clan's income bonus. Even a middling Hojo player could sweep the campaign map before the first European contact.
Kensai with decent Honour/Weapons/Armour can put down whole peasant rebellions, add a few to your uber-general's army and watch them carve everything up, excluding the Heavy Cavalry lead by heirs/Daimyos, Warrior Monks and high level Naginata Cavalry.
The Takeda clan will sometimes start with an heir who has Honour 5-6 in addition to their Daimyo with Honour 6.
Arguably, the Dismounted Feudal Knights are an early-game Game Breaker. These powerhouse infantry units have insane defensive bonuses and are very effective in close combat. They're eventually countered by crossbows and gunpowder. They're also incredibly vulnerable to a direct cavalry charge. Best used in an assault.
The Scots have access to Noble Swordsmen, who are basically Dismounted Feudal Knights on steroids. Also, the Scots have Highland Nobles which, when fully upgraded, cleave through most infantry and cavalry like a chainsaw through tapioca.
The Scots don't get high-end gunpowder units and have relatively weak archer units. The reason for this is because when you've got 1,200 screaming, painted, claymore-swinging Highland Nobles charging you, no amount of arrows or bullets are going to save your ass. Cavalry are squished outright by the range of pikemen that the Scots can field. Timurid Elephants? Psh. Scottish infantry are some of the best in the game, especially on a charge, and are pretty cheap compared with most other factions' infantry. Pair Highland Nobles with some Noble Swordsman for shock attacks and a bunch of pikemen to ward off cavalry or accept counter-charges, and you'll destroy nearly everything.
The Stainless Steel mod manages to make the Scots even more broken. The Highland Nobles are nerfed (while still powerful, they're harder to recruit), but recruiting pikemen is even easier now, so most Scottish armies are going to be packing at least four to ten units of pikemen. Pikemen are powerful here, particularly thanks to the combat system. Every time a soldier takes a hit from a weapon that doesn't penetrate their armor, they have a brief "flinching" animation that either stops them from advancing or prevents them from attacking. Pikemen, thanks to their long pikes, inflict these "flinching" animations well before an opponent can get into striking range, and three or four men at least are going to be poking every enemy soldier, thanks to the pikes' reach. This dramatically slows them down and makes it hard to attack them (just like it did historically) and with hordes of pikes poking them, even heavy infantry will eventually go down, and the small size of a cavalry unit compared to the pike unit ensures that every cavalryman is getting stabbed by dozens of pikes at a time. But the real kicker comes from the fact that Scotland is based on the British Isles, and all castles in the British Isles, once upgraded with the right archery ranges, can produce longbowmen. Pikes + longbows = nearly unbreakable formations that render just about any form of cavalry into a sad joke. Even missile cavalry are largely useless against this formation, as the longbowmen generally out-range them, outnumber them, and have better bows. So, instead of the Scottish armies in vanilla Medieval II, which are a powerful infantry-oriented force that can destroy anything on a charge, you instead get a nearly-invincible Scottish army with unbreakable pike-and-shot formations, about three hundred years before widespread firearms use resulted in its development historically.
Think Highland Nobles and Noble Swordsmen are bad? Try the Moors Dismounted Christian Guard unit. Sure, you don't get them until late in the game and all the other infantry the Moors can field are mediocre, but their baseline stats of 16 attack and 22 defense, low upkeep and good morale and stamina more than make up for it. The Moors also get Camel Gunners, which are essentially mounted musketeers, and start off close to Timbuktu, where a few merchants can make several thousand florins per turn.
Camel Gunners really are in a league of their own. The mobility and speed of light cavalry? Check. Sizeable stock of ammunition? Check. Highly accurate missile attack despite being a gunpowder unit? Check. Ranged attack that can tear through entire units of heavy infantry like paper? Check. Can fire on the move? Check. Faster reloading than other ranged units? Check. And enemy cavalry, who are the only units fast enough to pin them down, receive a hefty debuff due to horses being unnerved by the smell of camels.
In the Americas expansion, New Spain starts out with both Hernan Cortez and a couple of culverin artillery pieces. The culverin is able to pretty much one-shot any tower, wall, or gate and can blow massive, gaping holes in enemy regiments, and the native population has no possible counter. Cortez himself, meanwhile, has a ridiculously tough cavalry bodyguard and starts out with eight Command stars that only go up, and provides an immense morale bonus on top of that. In other words, Cortez can practically conquer most of the Aztec empire right off the bat with only his single army stack. Which is not too far from what happened in real life.
Any Islamic faction's imam can call a jihad if he has 4+ piety. In other words, if you have any reasonably competent imam, you can call jihads every ten turns or so. A massive army with major bonuses, no upkeep, access to almost free religious mercenaries, that can move faster than ships can sail and is available on demand? Yes, please.
A pretty good example of this is when playing as the Moors. You begin with a Imam that can immediately call a Jihad after building up a small army of 8 units. After hiring a bunch of Jihadist mercenaries, you can effectively destroy either Spain or Portugal by blitzing their inexperienced and lightly spread out forces and capturing their only 2 settlements. Wait ten more turns, then you can take out the other faction, and you immediately have control of the Iberian peninsula and your home in North Africa.
The downside is that you can only declare Jihad on former Muslim lands that have been conquered by other factions, so it's not too powerful unless you're on the ropes or, as has been said, are aiming to conquer Spain. However, Crusades can target any large non-Christian city. An army on the Crusade has no upkeep and can recruit lots of excellent and cheap mercs. You can have as many crusading stacks as you have generals. The downside is that the army must move towards the crusade target every turn or it starts deserting in droves. The catch here is that it doesn't matter whether you close in fast or slow, only that your army ends its turn closer to the target than it was before, even if it's only one square.
Cavalry against everything else, at least in the early period. It is possible to completely eradicate entire armies on Very Hard with just two units of General's Bodyguards, if properly applied. This includes armies with other, weaker, knights. Watch units of spear militia, the supposed counter to early-period cavalry, get smashed aside by your bodyguards with few losses for the bodyguards. Cavalry are a lot more expensive, but it still doesn't adequately reflect their awe inspiring power.
This is so bad that using large amounts of cavalry, or even worse, using cavalry almost to the exclusion of everything else, will earn you dirty looks in multiplayer. The first thing an incoming newbie learns about multiplayer is to ask about the policy with cavalry; usually the limit is six horse, up to two of which can be horse archers.
Normally the Papacy is an extremely annoying version of Goddamned Bats, but you can turn it to your advantage through some religious manipulation. If you're a Catholic faction, start the game by immediately allying with the Papal states, then as you work on expanding your empire keep spamming churches and priests, and send those priests to battle heresy in non Catholic lands. Building churches, recruiting priests, and battling heresy all increase papal favor (especially when you build cathedrals), and consistently recruiting priests from the same place opens up guild options. Soon you'll have high papal favor and plenty of guild trained priests experienced in battling heresy. If you get a guild headquarters you can usually get each new priest to start with at least five piety, the minimum to become a Cardinal, and it will only go up as you let them preach and denounce heretics. Since you have high papal favor, your priests are the first choice for new Cardinals, and with that you can easily get an entirely stacked bench (excluding the other two preferati), getting your faction members elected Pope every time. And with that comes consistent good favor for you and risk of excommunication from all of your Catholic enemies. If you need the extra push towards excommunicating them blowing up their religious structures works great. The downside is it will take a lot of money and probably at least a couple of generations, so you have to play your cards carefully in the meantime, but once you've accomplished it...
Horse Archers in Rome. They're practically impossible to catch with cavalry (and a clever player can simply support them with some melee cavalry) and can easily maneuver themselves behind enemy infantry units where they don't have their shields for protection. The only thing they need to be wary of are foot archers. The Cliblinarii of the expansion are even worse in that they're tough horse archers. Cliblinarii immortals are quite capable of mauling their way through several units of opposing Roman soldiers. Spears are no refuge from these armoured nightmares.
MOTHER. FUCKING. CHARIOTS. Any army that can use them will flood their armies with them. Hope you enjoy watching in screaming frustration as they drive full-pelt into your units, running them over and causing them to run merely seconds after hitting!
Roman legionaries set to autofire. No, seriously.
Selecting a city and typing "oliphaunt" into the romeshell produced an elephant unit that could win most any battle with enough of them (they had to be auto resolved, otherwise they took no casualties but took ages to kill anyone).
The berserker unit also qualifies. They can take down half a dozen men in a single sweep, have almost unlimited stamina, and are among the fastest foot units in the game when berserking. If you can get them into charging range, even a single unit of them will force the other army to make every platoon in the area either run in the other direction or take absolutely insane amounts of casualties. Even the feared armored elephants are at a disadvantage in an even fight.
Berserkers are arguably Awesome but Impractical. It's an expensive, late tech unit in a faction that is cash and growth starved early on, which means you generally have to expand to get it. It requires a specific temple be built, which is an inferior version of another one of your temples. By the time you can deploy them, getting them close enough to even do damage is potentially going to be difficult. Several units can also attack Berserkers with complete impunity because they can continuously attack them without any chance of the Berserkers catching up. From a tactical standpoint, since Berserkers have very poor support by the time they show up (the other troops Germania has aren't bad per se, but when you are going against high tech troops equivalent to the Berserker, they are very low end even for the specialists), it can potentially be very easy to set them up to be slaughtered. They might outclass most other individual units, but it is very easy to set up situations where most of the unit is killed on the approach and when they do get in range, they get slaughtered by several units attacking simultaneously.
General's Bodyguards are at the height of their power here. It is an entirely reasonable strategy when playing as the Gaulish or Germanic tribes to put all your nobility into one stack and march on Rome itself. Bodyguards not only have better base stats than any other unit in the entire game, barring elephants, but they also regenerate their stamina 4 times faster than any other unit, meaning they can run rings around anything but the lightest of cavalry. With flanking attacks and strategic lures, bodyguards can utterly annihilate armies that have more than a ten-to-one advantage in numbers. In addition to these ludicrous advantages, bodyguards also regenerate their losses after every battle with no additional cost, and because of their hilariously lopsided kill ratios will gain golden experience ranks very quickly, turning them from powerhouse units into nigh-invulnerable walking demigods that can send armies of elite units routing if they so much as ride towards them threateningly.
One of the more popular total conversion mods is Third Age: Total War for Medieval II. The modders have generally done an excellent job of implementing and balancing widely different factions. There are, however, some glaring exceptions that make sense in the lore but are still murder:
Dwarves are notoriously heavily armored and dangerous in melee, meaning both arrows and infantry are ineffective against them. They're slow, but seeing as how most Total War combat revolves around city sieges...
The High Elves can recruit an "archer" unit that not only has the longest range in the game and fires arrows which can kill most heavy infantry units through their shields, but also has armor and a melee attack greater than most infantry and can match the speed of the fastest cavalry. You might be tempted to scream at the cocky Noldar elves when cheap enemy infantry surge in on your very expensive and difficult to recruit archers. That impulse will likely go away after the first time they butcher and rout the orcs that hit their lines and then merrily execute the fleeing survivors with arrows to the back. The only thing slowing the High Elves down is that they start off in the campaign in a fairly poor position with limited expansion options. (Dwarves to the north, Eriador to the east, Rohan to the south). As a result, expansion as the High Elves is slow and you won't be able to build a big, potent economy for a long time, so your armies will be rather small, unless you're willing to go totally against the lore and start conquering the other "good" races.
The Silvan Elves as a whole. Even their basic Light Elven Archers have range and firepower comparable to most other factions' best archers, and the Sentinels that are their second-level archers are excessively long-ranged, hit hard and have excellent melee capabilities as well. Because of the versatility of their archers, it is possible to have the lion's share of any Silvan army be made up of Light and Sentinel archers, which will utterly butcher any army trying to charge them. Sure, their melee infantry aren't that great, but when 75% of the enemy army is already dead by the time the lines meet, and the archers are just as good in melee as the dedicated melee troops, they don't need to be. It's possible to destroy an entire enemy army that outnumbers yours two or even three to one, with trolls and wargs, while only taking a couple hundred casualties at most. Hell, the Silvans' ranged capability is so great that they can siege and assault small fortresses and villages without setting foot inside them. Casualty-free victories are not only quite possible, they're almost expected when playing the Silvans.
The only thing that beats the above archers are trolls, which will beat them with absolutely no effort or direction from the player. If you see trolls on the other side, regardless of what your faction or army composition is, resign yourself to losing 300 men off the top. Oh, and trolls are also as fast as cavalry and can even climb siege ladders because they are coded as infantry.
As in un-modded Medieval II, heavy cavalry are very powerful. The mod writers did a very good job of rebalancing cavalry, but even so a dedicated Rohirrim cavalry army is very powerful. By using hit-and-run tactics to repeatedly charge, your 30-man cavalry units can easily crush 100-orc infantry units, even Uruk-Hai Pikes. About the only thing that can beat heavy cavalry is Trolls (which crush everything, of course).
The Balrogs that spawn in Moria are, as you'd expect, quite capable of butchering entire armies of elite soldiers.
The Thera: Legacy of the Great Torment mod has one of these in the form of the Uruk Dominion. The Uruk-Hai (which are pretty much exactly like the ones from the Third Age: Total War mod) are given some truly terrifying stats, including high attack, high armor and two hitpoints each. Essentially, they're an infantry version of a General's Bodyguard that doesn't suffer the sustained combat penalties that all cavalry get. Watch in horror (or glee, if you're controlling them) as they charge an entire stack's worth of elite heavy infantry and archers, and go right through their arrows to crush the infantry without stopping. And heaven forbid they assault a city or castle, because street fighting is where the Uruk-Hai troops excel. Fortunately, they're still vulnerable to cavalry, gunpowder and siege weaponry, and sieges drain their numbers if they're on the defensive.
General's/Royal Bodyguards are already tremendously effective in vanilla Total War, but in Thera, they become a vastly different story once you start piling the faction-specific artifacts on them. Even a mediocre family member or general can become a terror on the battlefield when he's wielding Excalibur in one hand, Odin's War Axe in the other, flying three crusade banners, draped in the Cloak of Human Flesh, and carrying the Book of Morrigan and the Holy Grail. Entire armies break before these generals even hit their lines.
In Empire and Napoleon, frigates are this. The main reason is that Artificial Stupidity prevents the AI from being able to organize a trap or a proper chase. This allows the various frigates (which are notably faster than most ships, while still reasonably-armed) to lead enemy ships on a chase throughout the entire battlemap, while constantly turning to bombard the nearest chaser and then turning away to widen the gap again. This is similar to horse archer tactics in the previous games - fire at the enemy and retreat to a safe distance, over and over. Moreover, if any enemy ship is fast enough to chase you properly, the AI gets even more confused and will usually run right into chain-shot range (at which point a single volley will usually slow them to a crawl for the rest of the battle). Since ammunition is unlimited in naval combat, you can continue doing this as long as you have time left on the clock (and that can be set to "infinite"). Also, this tactic works best when you only use one ship (reducing micromanagement to a minimum). You can take on pretty much any fleet with a single frigate this way. And then you research 38-gun Steam Ships...
Truth in Television: When smaller ships (especially frigates and seventy-fours) acted in concert, they could be unstoppable. Look at the Superb at Algeciras, which tricked two Spanish battleships into blowing each other up (it was dark) or the Indefatigable and the Amazon on the 13 January 1798, who forced a far superior French battleship, the Droits de l'Homme, onto a sandbar where it was wrecked, at the cost of 900 men.
Don't forget the "Old Guard" from Napoleon. These guys may be expensive and late game only, but they can cause entire enemy formations to break and run just from being on the battlefield. This, combined with their ability to inspire friendly troops and ludicrously high stats make them one of the most feared units in the game. Used wisely, even just one unit of these guys can break an enemy flank and change the tide of battle.
In the Fall of the Samurai expansion for Shogun 2, ironclads utterly dominate against regular ships in multiplayer. This has less to do with the toughness and extra range of the ironclads, and more to the fact that explosive shells almost always set fire to normal ships in a single volley, and ships on fire eventually explode, damaging all ships near them.
In vanilla Shogun 2, it's possible to build a Monastery (or Jodo Shinshu Temple), a Naginata Dojo, and an Armorer (Encampment upgrade) in a province with a Master Armorer (Smithing); in fact, the Ikko-Ikki start with Kaga Province, which has master smiths and comes with a temple. The one weakness of standard Naginata Warrior Monks is that they're vulnerable to arrow fire due to their lack of armor, but this combination makes them more heavily armored than vanilla Katana Samurai, negating their usual weakness. Particularly for the Uesugi and Ikko-Ikki clans, this allows them to become the "nuke" in the the Tactical Rock-Paper-Scissors.
Shogun 2 also has the Black Ships, which would be weak trading ships inEmpireandNapoleonbut can obliterate whole fleets of native Japanese ships inShogun 2. They cannot be built but will occasionally show up sailing along the shores of Japan. They can be captured, but you will most likely lose a lot of ships doing that. Additionally, capturing one results in a huge hit to your economy, as it has high support costs. On the other hand, most naval battles after that become a cakewalk. The ship just has to be properly positioned before the battle to maximize its use as a floating gun platform. There are weaker variants that can be built, provided the player has agreed to deal with Europeans. Eventually, a higher-tier Japanese gun platform can be built, but it is significantly weaker than its Western counterparts and less maneuverable. On the other hand, Western ships depend heavily on wind.
The total conversion mod [[Warhammer Rage of Dark Gods: Battle for the Empire]] for Medieval II Kingdoms, which is especially popular in Europe amongst fans of the series, was always notorious for giant enemy stacks spawning right on your doorstep, with little to no time to prepare whatsoever. But massed artillery and magicians will make mincemeat out of those stacks. And it´s not like there´s few of those. On the side of the Conclave of Light, we have Imperial Mortars and Bright Wizards as well as the famously inaccurate but devastating Helstorm Rocket Battery, the kislevitian Uragan Mortar, High Elves´ Archmages and the incredibly powerful dwarven Anvil of Doom. On the side of the Forces of Chaos, there´s the sorcerers of Tzeentch and Nurgle, the Hellcannon (which also tremendously reduces morale), the Chaos Dwarf´s Earthshaker and Doom Rocket. Even Orks and Goblins get one in the form of Shamans of Gork and Mork. They tend to have very high upkeep and are fragile in close combat, but if properly protected and positioned these troops will win the battle for you easily ... or wipe you out if the enemy has them.
In FOTS, the entire Satsuma clan can qualify. All three of the foreign powers are on their side, their income is more steady than most others, they modernize faster than most other clans, and they have more favor with the Imperial household.
To elaborate: the Satsuma clan is in a very convenient position, which is a bigger deal than in Shogun 2. They have two coastal provinces in a corner of sorts, and they also start with a blacksmith (which is a HUGE boon to their military) as well as easy access to sources of iron, copper and coal. With said iron, they can quite easily upgrade their blacksmith to an iron works and then to a gunsmith relatively quickly, giving them a source of highly accurate infantry and artillery. With a gunsmith, basic line infantry have their accuracy boosted from around 30 to 50, which is nothing to scoff at. Things get really silly once they have access to foreign marines, faction-specific infantry and guard units. Spending the time to research 'Domain and the Realm' and then constructing a firing range in the same province as your gunsmith gives infantry and artillery trained there a monstrous +35 accuracy bonus. Combine these amazing military bonuses with the wealth generated by holding all of Kyushu, and Satsuma can easily become the most powerful faction in the game.
Artillery in FOTS. Artillery can easily take out eight hundred troops in most battles.