X Accuracy in the first games gave moves Swiftlike accuracy. This includes the OHKO instant death moves. The real dangers of this are because beating the eighth Gym Leader, Giovanni, got you the TM for Fissure, which could be used on a lot of final form Pokemon, like Dugtrio, who could outspeed most Pokemon easily, and because OHKO moves could be used to defeat higher-level opponents, it made defeating the Elite Four (barring Lance, who could be defeated using a good Ice Beam user, which most Water Pokémon are) a piece of cake. The combination of X Accuracy and OHKO moves was so powerful that the mechanics for OHKO moves had to be changed in future generations so that they could never connect against an opponent whose level was higher than the user's level or increase in accuracy.
The Psychic-types were notoriously broken in this generation. They only had a weakness to types of attacks that did below-average damage even super-effective, and those moves were available to very few viable Pokémon. Due to a bug, one of those types actually didn't affect Psychic-types at all, rather than do double damage. Making things worse, the most common type was the Psychic-weak Poison-type. The Psychic-type advantage was augmented by the Special stat governing both Special Attack and Special Defense. This meant that Pokémon with high Special, such as Psychics, were much more useful than Pokémon with low Special, such as Fighting. This issue was fixed in Pokémon Gold and Silver in later generations by separating the stats and introducing two new advantageous types.
Fifth Generation maingame
In the Pokémon Black and White main game, the Lucky Egg item. In previous games, this was an extraordinarily rare steal from Chansey, but in this game, it is simply given to you free of charge about halfway through. It greatly increases the experience of the mon holding it, allowing you to gain levels at ridiculous rates provided you kept going through the tedium of redistributing it every time you switched your active Pokémon — of course, it's totally worth it to outlevel the Elite Four without any extra Level Grinding on Victory Road unlike the previous gens, and turn the game between getting the Lucky Egg and the Elite Four into one giant Curb-Stomp Battle.
A big one was the option for players to download both Mewtwo level 70 AND Arceus level 100 into Black and White from the Pokémon website. Have those two both in your team, and most of the in-game opponents don't stand a chance. Admittedly, the second battle with the Elite Four and the battle with Alder (their 'mons are all in the 60s and 70s that go around) still require some potions on hand, but still, it's way easier than it'd normally be. And you were able to get Reshiram or Zekrom as well, depending on your version.
Sixth Generation maingame
If you thought the Lucky Egg in Gen V was a Game Breaker, just wait until you see the Exp. Share in X and Y. Remember that crappy Exp. All from Gen I? Picture that, except it's actually GOOD. Long story short, it's a key item that while you have it on, the rest of your party gains EXP equal to half of what you had out gained. This item ensures that you won't know the meaning of the term "underleveled" before the second Gym. In fact, you won't even know the meaning of "grinding" for the rest of the game for that matter.
You could be training your lower-leveled Pokémon while making no effort to train your Starter, and your Starter still levels up at a rapid rate. One could essentially get away with training more than six mons and not suffer because of the speed, making it perfect for Nuzlocking.
For that matter, the Lucky Egg is back. It's possible to break the level-grinding even MORE by giving it to one of your Pokémon without turning the EXP Share off, letting them grow twice as fast without even being in a battle.
Combine those two with a Sweet Scenter to trigger repeated horde battles and...woosh. Prepare for your lead to get 3 or 4 k or even up to 5 or 6 k at times per 5 kills and your other mons still get a ton. You may need a decent supply of healing items or to remain near a town due to things like Spinda hordes spamming Confusion and Hypnosis and Weepinbell hordes who love poisoning you, but it's still worth it.
Capturing wild Pokémon now gives you the same experience you'd earn for defeating them. Not only do you no longer have to sacrifice potential experience to complete your Pokédex or expand your team, but with the right resources, capturing a Pokémon can be quicker and easier than defeating it.
Pokémon-Amie can turn into another serious Game Breaker if you use it enough. You bond with your Pokémon by petting it, playing minigames with it, feeding it food and such. While that might seem like little more than what's needed to get Sylveon, if you get your Pokémon's affection high enough, you'll receive epic side effects in battle, such as gaining 20% more EXP in battle, surviving what would be killing hits, avoiding attacks, and shrugging off status conditions. You'll be shocked and flattered when those effects happen in battle.
Pokemon Mystery Dungeon
The Pokémon Mystery Dungeon spinoffs aren't immune to this either. Most of the Game Breakers in the first two games (Red Rescue Team and Blue Rescue Team) were nerfed in the later games (Explorers of Time/Darkness/Sky), however. A short list:
Due to a programming oversight (no Pokémon in the first games was given a gender except for the main character), moves that caused the Infatuation status (Attract and the ability Cute Charm, at that time) were absurdly powerful, basically being a nigh-effortless way to ensure that enemies almost never got a chance to attack you. Not to mention the Unfortunate Implications of makingeverybodyfallin lovewith you...
In Gates to Infinity, the status can affect everyone once more... but it's somewhat fixed because it properly has the 50% chance of preventing attacking.
Due to the special properties of the Pokémon Castform and its signature move Weather Ball, a single Castform could do enough damage to KO any non-boss Pokémon in a single hit at low-to-middle levels, and at high levels, could even do the same to any boss Pokémon that didn't resist Fire or Water. This was only minorly nerfed in the Explorers games; Castform now has a 'size' of 4, meaning only one can be on a party and at the expense of another Pokémon, but most normal dungeons don't need more than the single Castform at one time anyway.
Upon maxing their IQ (the Pokémon Mystery Dungeon equivalent of 'Happiness', boosted by eating Gummi items), Pokémon acquired the ability "Super Mobile", which allowed them to travel over water, lava, clouds, and even allowed them to bash through walls to make their own path with absolutely no penalty. In a roguelike game, this naturally causes most of the difficulty to soar right out the window. In the Explorers games, this ability was severely restricted to a single legendary Pokémon, Palkia, whom you can only obtain after the game's storyline, making it more of a Bragging Rights Reward.
Also, several IQ skills on the right IQ groups to boost HP, PP, and other stats could be further combined with Three-Star "Exclusive Items" that buffed a particular Pokémon each (generally giving them Absorption against a type they're usually weak against, or putting them in permanent Light Screen/Reflect status). Not only that, but the effects of those items are shared among all Pokémon belonging to the same evolutionary branch! So for instance, Pokémon like Gallade, Hitmonchan, Vileplume, or Politoed could not only gain the buffs of their former evolutions, but also those of their counterpart(s), for a total of 4 buffs. And the Eevee family...
Also, while in the Rescue Team titles you had to combine specific items to net one, usually not even resulting in what you exactly wanted, in Explorers of Time/Darkness/Sky, you can look up the item you're looking for in the shop, and swap any exclusive items you want to get rid of for the one you want, basically turning any Pokémon with a broad enough "family", through IQ grinding and item gathering, into an Infinity+1 Sword!
Want to top even this? Then bring along also the two-star equivalent of the Exclusive Item to get an even bigger stat boost, make your 'mon binge on stat-rising items like Life Seed and Iron, and keep in mind that some of the rarest Exclusive Items affect all the 'mons of the same Type. Pair it with a double-type, and...you know the drill.
Multi-Hit attacks (Bullet Seed, Fury Swipes/Attack, Pin Missile). Due to the damage calculation in the Mystery Dungeon games being much different than the mainstream titles, these move actually hit as hard as most other attacks per hit. The brokenness comes from the fact that the STAB bonus from the mainstream titles is also implemented in the Mystery Dungeon games, meaning with the right Pokémon (A Treecko with Bullet Seed and the Concentrator skill for instance), this can be quite lethal.
Speaking of Bullet Seed: because of how damage mechanics work, it can perform up to five ranged attacks, each of which deal a great deal of damage by themselves. And if you happen to KO the 'mon in front of you? It keeps attacking the ones behind it until you either run out of moves or targets.
If you think that's bad, try linking Screech with a multi-hitting move, like Meowth or Persian with Fury Swipes, and watch how fast you take your opponents down.
In the original game, Protect made you invincible to damage for one turn, and Sonicboom only did 20 points of damage regardless of the Pokémon's stats. In the Mystery Dungeon games however, Protect now stays in play for multiple turns, allowing for multiple attacks while the opponent is helpless, and Sonicboom is a ranged attack that hits for 55 points no matter what. Also, unlike the original games, where most Pokémon could easily have 200+ health points at the end of the game, most Pokémon naturally won't even reach triple digits (100+ health points) at the later points of the game, so just imagine how devastating and frustrating 55 direct points of damage from a distance (especifically off-screen) would be.
Considering that in the sequel, Pokémon possess their two abilities at the same time, and that one of its abilities (Levitate) negates one of its only 2 weaknesses while the other (Heatproof) negates the other, Bronzong is a top-class Game Breaker. Its awesomeness is greater when you consider that Bronzong already has high defenses to go against the other attacking types which Bronzong is not weak to, along with the fact that his moveset is very good with Psychic and Gyro Ball being only some of the great moves it can learn. Add to this mix the fact that his low speed from the main game series isn't a factor in these games, and you'll have no trouble getting through most places in the game.
There was also Shedinja from the Pokémon Mystery Dungeon series, who unlike in the regular games where they were limited to 1 health point no matter what, were able to eat Sitrus Berries in order to increase its health maximum. Couple that with the Wonder Guard ability, and the Super Mobile ability in the first game which is obtained by eating enough Gummies to reach maximum intelligence, to see how destructive one Pokémon could truly be.
Attacks that hit the entire room are usually game breakers, i.e. Earthquake. Who cares if you wipe out your teammates (Or, maybe not, considering the hit radius is ONLY an entire room and teammates in corridors are safe) if you can curbstomp an entire Monster House? Or, for even more mileage, you can link Earthquake with an ability like Protect. Wipe them out, then defend yourself! The worst part is, a single Pokémon can have a multi-hit ranged move, a defensive move, and a room-clearing move, and will likely not find much use for any of their moves because that Pokémon will likely be stat-grinded enough from feeding it Gummis that its generic attack one-hit KOs anything that's not a boss.
Pfft please, if you want a nasty one, try Heat Wave, which hits all the enemies in the room AND leaves your allies unharmed. Same with Silver Wind, or any attack that hits the room all at once. On top of that it can be linked, and Charizard has it from a VERY LOW level, making dungeons that force you back to level one to be a snap with a Charizard on your team.
If you're talking about room affecting moves, specific mention has to go to Agility which raises all Pokémon on the same team's speed greatly, and moves like Silver Wind which hit the entire room, and can power up all of the stats of the one who uses it.
You can't forget the abilities Chlorophyll and Swift Swim, which in the main games boosted speed under Sunlight and Rain (respectively). However, since speed doesn't work in the same way in these games as it does in the other games, they had to figure out some other way to have the ability manifest itself. Of course, this comes in the form of having your Pokémon attack twice in the same turn for the PP cost of ONE attack. Now combine this with either Groudon or Kyogre, an already broken move like Heat Wave, and the fact that Fire/Water moves get a boost in that weather...
Then there's Mirror Move. In the mainstream games, it's simply a move that allows you to replicate your foe's move; rather gimmicky and not very useful. In Mystery Dungeon, however, it acts as a Protect—while simultaneously reflecting every move that hits you back at the foe (including status-inducing moves). Not only does this last for several turns, but the reflected moves also factor in types. If you started with a Torchic, simply use Mirror Move on Palkia and watch it OHKO itself with a reflected Spacial Rend. Fantastically useful in monster houses and any multi-boss fights.
In Gates to Infinity, once you're given the ability to form a Team, there's three additional starting candidates, in addition to your character and your partner: Emolga, Dunsparce, and... Virizion. Virizon is an Olympus Mon with appropriate stats, a combination of types you won't find pretty much anywhere else, and, oh, is at level 20. You're probably in the early-mid 10s at this point with basic-form starters. Do the math.
Excadrill are just as overpowered here as they are in the regular game. They learn both Swords Dance and STAB Earthquake at a relatively low level, and their pre-evolved forms aren't that difficult to find (they can be found in the first dungeon). Combine this with its already ridiculous Attack, and you've got something that deals serious damage, even to Flying-types. They're so stupidly powerful that, unless you know what you're doing, your team's Excadrill will one-shot everyone, including your teammates. And if that wasn't enough, Sand Rush allows the user to move twice in one turn if sandstorms are brewing. Dungeons that force you with only one party member? No worries, just send in the mole. It can make short work of just about everything. The only things it can't handle are the Path of No Return and Slumbering Cave dungeons, since they force all Pokémon to level 5, and their movepools are reduced to level-up only.
Companion Mode is quite powerful here, if you're patient enough. When in Companion Mode, the story stops completely, and you get to play as a party of the Pokémon you've recruited. Here, you can build up Pokémon Paradise and accumulate massive amounts of resources and items, at the start of the game! The only limits to this are your patience and the fact that better dungeons/missions will be unlocked as the main story is progressed. What really makes this powerful is the fact that there is a shared experience system in effect. All EXP that is earned is shared by all Pokémon, even the ones in Paradise and the story characters. With enough time, your story Pokémon will be vastly overleveled.
Pokemon Rumble U, if you actually pre-ordered it, comes with Black/White Kyurem. These two, once you get their passwords in the game, are unstoppable killing machines that kill anything bar bosses in a single shot. That is, until the further levels, but it's still amazingly easy.
Pokemon Rumble Blast (the 3DS version) also has a password system, accessible after the second town. Taking a few minutes to look for codes on the internet and entering them gives you steadily increasingly powerful Mons depending on the area. They are all at a set level and usually know moves you couldn't learn until Axle Town through the random move machine. All you have to do after entering the code is to find them in the overworld and defeat them, and for their level and obtain point in the game, they are absurdly powerful(read: Victini/V-Create, Tornadus/Hurricane, Zoroark/Foul Play). And once you beat the game, you can access more codes for Olympus Mons like Groudon, Dialga, and Lugia. For free.
And if you were patient enough to continuously beat levels to access that 5% chance, one of many Olympus Mons is yours, depending on the level. Most are faithful to their main series counterparts (Darkrai having high Attack, low Defense, or Lugia having low Attack but high Defense), and some even have both maxed out. And most come loaded with their Signature Move from the start (or a high power attack if they lack one), just to make the rest of the game, barring the EX Battle Royales, a complete joke. And even then, if you do beat the EX Royales and keep searching through specific levels... say hello to Arceus and Mewtwo.
Trading Card Game
The Game Boy Color version of the Pokémon Trading Card Game, faithful to the original game, had hundreds of cards that relied on a coin flip. For example, the Poké Ball card allows a player to search their deck for any Pokémon and put it into their hand, but only if a coin flip comes up heads. This was not in itself unbalanced, but the problem came when the game's coin flipping system was not randomized. A player with a good sense of timing could earn a favorable result on every coin flip. This made cards like Vileplume◊ and Kangaskhan◊ able to destroy an opponent in just a few turns.
On the subject of the Game Boy Color version, Imakuni? was a Game Breaker. He was weaker than all the AI characters, but if you defeated him (which was quite often), he gives you an unheard-of four booster packs. This makes racking up cards rather easy.
There's also Pokémon like Hitmonlee◊ who could attack your lead Pokémon with a powerful move, or your benched Pokémon for 30 points of direct damage, easily defeating first form Pokémon with really low health in one or two turns, while it had enough health to last for a while if you didn't have a powerful psychic Pokémon to counter it.
Similar to the above example the "Haymaker" cards which are basic Pokémon cards with HP comparable to most evolution cards and low energy costs; Scyther, Hitmonchan, and Electabuzz can give you a massive advantage at the beginning of a match, and then there's what happens if you have four in a deck...
The Pokémon Trading Card Game underwent Sequel Escalation for each generation that was released, plus another for HeartGold and SoulSilver. It's reached the point where the old Haymaker deck is easily overpowered. This is due to an increased emphasis on evolution in that evolved Pokémon get lower Energy costs for greater effects, more Pokémon capable of hitting the Bench, vastly improved Trainer cards (since then split into Trainers, Supporters, and Stadiums) that can easily search for cards of your choice from the deck or discard pile, and a trend away from Energy being the requirement for attacks, instead leaning towards other effects on the field. Together, this has resulted in a frenzy pace, most tournament decks refined to be fully set up in 1 turn, or 2 at most. For instance, Gyarados has an attack that does 30 damage for each Magikarp in the discard pile. A popular deck is designed to use Rare Candy to evolve a Magikarp on the first turn (bypassing a rule prohibiting this), find the 3 remaining Magikarps and discard them, attach an Expert Belt (which boosts attack power by 20), and use this move for 110 damage. Also, this attack requires no Energy at all. Compared to decks like these, the Haymaker is but a light jab.
The Team Rocket booster packs, full stop. These had such Pokémon as Rocket Blastoise who could use its "Rocket Tackle" attack for 40 damage, and flip a coin which if it was called right, negated any damage from the opponent in the next turn including status effects, all for reducing its health by 10 HP, Rocket Charizard who had an attack called "Continuous Fireball", which for one fire energy it could flip a coin until it got tails, and the attack did 50 points of damage to the opponent for each heads it got, and the best part is that most of these overpowered cards were common cards. The rare cards were even worse like Rocket Dragonite who had a power that allowed you to put two Pokémon on your bench at once while it was in battle, and a Trainer Card called "Here Comes Team Rocket" which made both players play the game with their prizes face up, and eliminating the random possibility of picking a weak prize after a victory while thinking of a proper way to use your prizes in any order you want, while keeping an eye on your opponent's possible prizes at the same time.
And as if that's not good enough, Generation V gave it Psystrike, a signature move that has good base power and hits physical Defense with Special Attack. It is so awesome that it is now THE one and only Pokémon in the metagame that has an excuse to run its previously unseen Psychic STAB move without any drawback. It no longer needs to sacrifice itself to beat Blissey, and the commonly used Pokémon in ubers is not Psychic or can't take Psystrike. Mewtwo also has Aura Sphere, Shadow Ball, and Fire Blast to cover the coverage to hit everything you need to hit. Finally, factoring Stealth Rock, it can hit everything he previously needs a specific move to hit with enough damage especially with Calm Mind up. Psystrike gives him an excuse to run STAB, and ability to beat Kyogre and Blissey more reliably with his Calm Mind set.
Perish Trapping (using Perish Song to faint the opponent in 3 turns and Mean Look/Spider Web so they couldn't escape and nullify the effect) was a very effective strategy due to the primarily defensive nature of the metagame. Combined with a sleep-inducing move, non-offensive Pokemon would be rendered helpless against their imminent demise, which led to sleep-inducing and trapping moves being banned on the same moveset. The faster pace of the metagame in later generations would render this ban unnecessary and limit the effectiveness of Perish Trapping in general, but this is likely why Misdreavus, the most notable user of the strategy, could no longer learn Hypnosis in later generations.
Ruby and Sapphire gave Wobbuffet the Shadow Tag ability, which prevents the opponent from switching, and Encore to its movepool. Forcing the opponent with Encore to repeat their moves while preventing them from switching is possibly one of the most sadistic strategies Pokémon has to offer. If the opponent decided to play around this by using a non-damaging move like Swords Dance, Wobbuffet would happily switch out to another party member, which then had a free turn to set up any stat buff it wanted, while the opponent was still Encored and helpless. And by giving Wobbuffet the Leftovers itemnote which gives it 1/16-of-maximum-HP recovery every turn (note again that Wobbuffet has near the best HP in the game), it could stay on its feet almost indefinitely. This gives Wobbuffet the dubious honor of being the first non-legendary Pokémon to be banned from competitive play. And the second? Wynaut. Wobbuffet's baby form. If that wasn't bad enough, imagine two Wobbuffet. both having Leftovers, facing either other. Neither can attack except with Struggle (and remember, Wobbuffet also has rock-bottom attack power), and neither switch out. Literally, a battle no one can win. This actually caused Nintendo to change the rules of official tournaments, first banning use of Leftovers on Wobbuffet, then changing the format of all subsequent tournaments to doubles, where Wobbuffet is mostly dead weightnote it'll prevent opponents from switching out, but since it can't force opponents to target it, an opponent will focus their attacks on Wob's partner, rendering its countering abilities moot while forcing its partner to fight two-on-one.
Kyogre gets special mention because unlike Groudon who's a Ground-type Pokémon with an ability that raises Fire-type moves, Kyogre's a Water-type Pokémon that has an ability that raises water type moves, and boosting its Water-type moves along with STAB!Oh, yes, and it can learn Thunder, which always hits during rain! Combine that with the fact that its Drizzle ability is an instant Rain Dance on the fieldnote which, until Generation 6, was permanent until someone else changes the weather and Thunderbolt for opponent Water-types and Ice Beam for any dragons that try to resist its attacks. Consider that its Special Attack are on the same rank as Mewtwo with considerably better bulk, give it a Choice Specs to boost its Special attacks further, or Choice Scarf to raise its speed to the point where only a few Pokémon without Choice Scarf itself could exceed it, to see how devastating it is in battle. Simply put, Kyogre is the Stealth Rock of Ubers: you MUST have a counter for it if you want to win. This Pokémon IS GameBreaker incarnate in its own tier. So much that you might see ComMons such as Quagsire, Lanturn, Shedinja, and recently Gastrodon in Ubers just to beat Kyogre.
Groudon is no joke either. 150 base attack, and 140 base defense paired with solid 100 base HP makes it a very sturdy tank that even Rayquaza's Outrage can't break easily. He has a crazy movepool including Thunder Wave, Stealth Rock, Rock Polish and Swords Dance. Oh and he's a Ground-type meaning he gets STAB Earthquake which is awesome and is immune to Thunder Wave. In terms of power, if you, by some case, let Groudon set up more than 1 boost, you're going to face probably the most unstoppable physical sweeper in the whole game.
Rayquaza. A Flying Dragon with 150 offense on both sides, at a downside of a not so high 95 speed and decent (but low by Uber standards) 90 defense and Special Defense with access to the dreaded Swords Dance + Extremespeed combination and Dragon Dance. It's able to become a Wallbreaker as well, thanks to its Dragon typing. As a bonus, its weather nullifying ability means it's unaffected by numerous factors that make Wallbreaking it harder, particularly Rain that weakens Fire type moves for Skarmory. There's a good reason it's recommended to pack a Steel type in the Uber metagame. And even that might not help at all, since the Rayquaza from a certain event gets V-Create. Yes, V-Create, Victini's signature move which runs off 180 base power. You may now become petrified in abject horror.
Generally speaking, in the fourth generation, Dragon-types were heavily buffed. The new move Draco Meteor, a 140 base power special Dragon-type move was learnable by every fully-evolved Dragon and basically one-shotted anything not a Steel. Outrage was also buffed, becoming one of the deadliest attacks in the game in Generation IV with an increase in power from 90 to 120. With the advent of the physical-special split, Dragon Claw and Outrage went from being special moves to physical, letting Salamence, Dragonite, and Garchomp run STAB Outrage off their superior Attack stat. The extremely popular buffing move Dragon Dance, widely distributed among Dragons, could now be used to boost Dragon-type moves. Dragon-types other than Garchomp could now run their choice of physical, special, or mixed attacking sets, which undercut their traditional counters, especially when factoring the new Choice itemsnote boosting one of Attack, Special Attack or Speed by 50%, at the cost of freedom to switch moves By the end of Gen IV, Garchomp, Salamence, Latios, and Latias were all Kicked Upstairs into Ubers, resulting in Dragonite and Flygon being the only real such threat left in Gen IV Smogon standard.
While Garchomp is mentioned above, it really deserves an entry to itself - it's easily one of the most powerful non-legendary Pokémon ever created. It possesses an excellent Attack, one of the best possible offensive typings in the gamenote before Generation 6, there were only three Pokémon (Bronzong with the ability Levitate, Shedinja, and Skarmory) who could resist both the Dragon and Ground type attacks Garchomp gets a STAB-boost for, all of which are weak to Fire attacks Garchomp can also use, very good Speed, and more bulk than Swampert (a defensive Pokémon). The flagship Garchomp set contained the Yache Berry, which cut the effectiveness of Ice-type attacks, Garchomp's only major weakness. This allowed it to get a Swords Dance off and guaranteed one or two KOs before Garchomp goes down. An even more hated version was BrightPowder-Chomp which, combined with the Sand Veil abilitynote opponent accuracy is cut to 80% in a sandstorm, ever-present in this generation, and Brightpowder subtracts an additional 10% meant that with just Tyranitar in your team, attacks hit him a maximum of 72% - which you could happily abuse with Substitute until you dodge an attack, Swords Dance, and sweep. This is all made even easier because Garchomp can switch-in and force out most Pokémon because of how impossibly threatening it was.
Palkia. Extremely powerful STABs, capable of running a mixed set, only two weaknesses (Dragon and Fairy), wreaks havoc if used with Kyogre. Only three Pokémon note Empoleon, Shedinja, and Ferrothorn can resist both of its STABs, only the last of which is used at all in Uber play. It can even check Kyogre with its double resistance to Water-type attacks. Blissey? Palkia used Aqua Tail. Groudon's Drought? Won't live that long with an average (by Legendary standards) Special Defense. Shedinja? Plenty of options to take down the little husk.
Dialga. It's for a reason why he's almost always a top 5. Awesome typing that resists Stealth Rock and is immune to Toxic and Toxic Spikes, awesome movepool, and it even learns Stealth Rock and receives a very great stats spread offensively and defensively. This thing is basically a Droughtless Groudon with Dragon typing.
Arceus also deserves a mention. A 120 base stat spread in everything, a movepool only rivaled by Mew and Smeargle and the ability to change its type by holding elemental plates makes it a very versatile and dangerous Pokémon that is able to duplicate almost every Pokémon in the game and do the same roles with the same level of efficiency all in one package. It effectively rivals Kyogre in term of brokeness.
To show how insane Arceus is statistically, see Kyogre and Groudon (the two most common Pokémon in ubers that are overall the best Pokémon) and their stats. Kyogre and Groudon only win in attack and their respective defensive side (physical and special respectively). But on the attacking side, Arceus has 120 Speed on its side, significantly faster than both since that means even with very little EVs, Arceus is faster than either with the maximum speed they can reach. It also means the resident Stone Wall Lugia is slower than Arceus. On the defensive side, despite the differences, Arceus is only slightly less bulky on the weaker defensive stat. So little that it's no matter at all. Oh, and did we mention that by forgoing some hold item it can change its type AND getting boosted STAB at the same time? Let's not start talking about its notable moves.
Darkrai is brutal itself. 135 Special Attack, while not as ridiculous as some of the other things it'll be dealing with, is still very impressive, and 125 Speed is a godsend in the comparatively-sluggish Ubers meta. Its movelist, however, is what pushes it over the edge. First off, it has a very useful Secret Art called Dark Void, which is an 80% accuracy Sleep-inducer (most sleep moves have far worse accuracy). Coupled with its excellent Speed, there isn't much in Ubers that it can't get the jump on, and its Ability causes damage every turn to sleeping foes and acts as a weaker-but-free Nightmare. It is also retardedly versatile, having a vast array of viable moves to choose from that will always leave someone wondering what it's running; guess wrong, and you're out at least two or three Pokemon. Its main weakness is its frailty and vulnerability to revenge killers, but it offsets both by being able to cause enough damage to create an insurmountable advantage by the time it has been killed.
In Generation V's metagame, one playstyle on online simulators provoked quite a bit of outrage; Rain teams. See, thanks to the Dream World being able to unlock Pokémon with unique hidden abilities, Politoed now has Kyogre's Drizzle ability, summoning permanent rain storms. Combined with the speed-doubling ability Swift Swim and Water's fantastic defense and neutral coverage, the metagame was completely annihilated by a blitz of obscenely fast, strong, and bulky water Pokémon, with Kingdra, Kabutops and Ludicolo being considered Uber-level fighters. Add to that Manaphy, who can maximize its special attack with two turns of setups and is immune to status in the rain (letting it instantly heal with Rest) and it's no surprise that Rain was completely dominant in the Generation V metagame until Smogon declared it illegal to utilize Drizzle and Swift Swim together.
Another Generation V Game Breaker banned by Smogon is the Moody ability, which causes one stat to be sharply raised (essentially a free Acupressure boost) while another drops one stage every turn. Despite appearing on otherwise overlooked Pokémon such as Bibarel, a few turns of stalling can result in an unstoppable, Nigh Invulnerable Pokémon capable of demolishing an entire opposing team, especially considering one of the stats that can be boosted is Evasion. To quote Smogon:
Bidoof nearly had to be banned because of this ability. Bidoof.
A newcomer to the Generation V uber list is Blaziken, so far the only starter to make it to that tier. Its new Dream World ability of Speed Boost raises Blaziken's normally mediocre speed by one step every turn (and it can use Protect to guarantee that first crucial turn), and the retooled Hi Jump Kick grants Blaziken a devastating STAB move that doesn't lower its stats. Add Stone Edge or Shadow Claw for coverage, and there are few things that can switch into Blaziken without promptly being obliterated. Its biggest power is the fact that you need to deal with it FAST. Even with that, it has a high chance to cripple a team with its massively powerful STAB attack.
Despite the fact that Generation 5 brought more ways to check Garchomp, it got banned. Again. Even with checks such as Air-Balloon Heatran who could otherwise come in on any attack and take it out with an Ice-type Hidden Power, if Garchomp was hiding behind a Substitute, it didn't stand a chance. In fact, the combination of Substitute and Swords Dance in a sandstorm was a menace to anyone who couldn't break the substitute and take out Garchomp before it boosted too much or set up another substitute. Unlike in Gen 4, in which Sand Veil was the straw that broke the camel's back, Sand Veil and the Luck-Based Mission nature of this set were entirely the reason for Garchomp's ban this time, and once it got Rough Skin and Sand Veil itself was banned, it was allowed once again.
Excadrill. It has one of the highest Attack stats in the game, and one of the best STAB types, though it appears to be balanced out by its mediocre Speed. However, its ability, Sand Rush, doubles speed in Sandstorm. Yes, DOUBLE SPEED. In fact, it reaches speeds so high almost nothing can outrun it, and it hits like a truck too. Oh, and the Pokémon with instant Sandstorm setup, Tyranitar and Hippowdon, are also quite the Game Breakers themselves (though Tyranitar is far more common), so Excadrill has plenty of chances to go crazy. Also, it can also learn Swords Dance; let it get one off, and you're headed for a Total Party Kill. Priority is pretty much the only thing that can stop it. It should come as no surprise that it got banned.
Genesect got Kicked Upstairs into Ubers quickly after its appearance in 2012. With high Speed, Attack and Special Attack, respectable defenses, and an automatic 50% boost to either Attack or Special Attack when it switched in (depending on whether the foe's Defense or Special Defense was lower), it was the most widely used Pokemon in OU during its heyday. Genesect's standard builds got at least neutral coverage on all of standard OU and Ubers, so there was no such thing as a surefire counter to it.note The only Pokemon in the whole game that resisted Genesect's standard movesets, Rotom-Heat, lacked the defenses to wall Genesect or the offenses to do much of anything else. It was also Stealth Rock weak in a metagame, where Rapid Spin was not easy. Another thing that adds to the trouble of dealing with Genesect is its access to U-turn, which allows it to escape from a potential threat to it or a counter to something that deals with the threat, and Download allows it to hit hard with U-turn even without investing in attack, thanks to STAB, Download, and attacking statsnote it has 120 base Attack and Special Attack, but its physical movepool is not as good as its special one, so purely physical Genesect sets are uncommon. Heatran, which would other wise be a nearly flawless counter to Genesect, risks being smacked by U-turn, possibly losing its Air Balloon, and Genesect switching out to, say, Dugtrio, who proceeds to destroy it.
While in lower tiers it suffers from no good abuser (as of Gen V, only 2 abusers are really great and the other abusers, while good, are no match for rain abusers), in uber tier sun (that is summoned by Groudon) has many great abusers such as Ho-oh and recently Reshiram in Gen V. Kyogre might be better on a single comparison but in terms of a supporting role, Groudon outclasses Kyogre with both its ability and movepool.
A major difference in Gen IV and V, and even somewhat in Gen III compared to the first two generations, is that with the official tournament focus shifted to non-single battles, now Pokémon and abilities and moves are not really balanced toward singles 6-on-6 as they were before. This leads to stuff that, while fine for doubles/triples, where a mon risks being a focal point for attack which could minimize its impact, in singles that same mon becomes really powerful since it can last longer in singles to have a bigger impact - like the Moody example above.
White Kyurem is a monster when allowed to attack. A base Special attack of 170 means that depending on the set, Kyurem-W can obliterate virtually anything with the right move. Between Draco Meteor and Ice Beam, Kyurem-W can take down many defensive threats, say, Lugia, Dialga and Kyogre. It gets better; give Kyurem-W some Choice Specs(boosted Sp Atk in exchange for being locked into 1 move), and most defensive threats fall to a single attack. It's only setback is it's unfortunate secondary typing, making it vulnerable to all entry hazards, Fighting and Steel type moves which are common in the Ubers metagame, but it is still a massive threat, more so than Black Kyurem.
Pokémon X and Y has the cover legendary Xerneas, which is already being looked as on par with Kyogre and Mewtwo in power. Its signature move, Geomancy, essentially gives two Quiver Dance boostsnote increases to Special Attack, Special Defence and Speed after the end of the turn after which you use it. Combined with its bulk and its Fairy-type (a fantastic typing in Ubers, both defensively and offensively), you have a frighteningly powerful set-up sweeper that is hard to take down. For even more curb-stompy goodness, you can give it a Power Herb to make the set-up instantaneous! There is one downside, though; if your Geomancy'd Xerneas is somehow switched out, say goodbye to your sweeper and enjoy having it as dead weight for the rest of the match.
Ho-Oh made out like a bandit this gen as well. Thanks to Defog being able to sweep hazards off the field without being blockable like Rapid Spin, Stealth Rock has been defanged even further; incidentally, it was pretty much the only thing keeping Ho-oh from exploding in Gen V, and now that that's gone... whoof. With monstrous bulk (particularly Special Defense, which is a godsend in the Special-centric Ubers meta), excellent offense, a wide variety of options that makes it very difficult to read, a fantastic Secret Art that hits like a nuke, has no real drawbacks, and can easily cripple enemy physical attackers with a 50% burn chance, a godlike Hidden Ability that essentially gives it consequence-free Brave Birds and allows it to run Choice sets now that it can heal without Roost, and, as of this gen, a Fairy resistance, making it invaluable when dealing with Xerneas and Arceus-Fairy, Ho-Oh is pretty much perfectly designed to wipe out all but a few select members of Ubers in its rainbow-feathered rampage.
Mega Blaziken receives sizeable stat boosts in offence and speed, though it is slightly weaker in sheer damage output than a Life Orb Blaziken. A much bigger change is that, thanks to changes in breeding mechanics, Speed Boost Blaziken now has access to Baton Pass. This not only allows it to pass both Swords Dance and Speed Boost at once, it is much harder to counter as Blaziken can very well escape to a counter-counter with all of Blaziken's boosts. It says something that high-level battlers in Smogon were so terrified of it that they basically quick-banned it without a vote in the early XY metagame, making Blaziken Uber for the second generation in a row.
Special Mention goes to Mega Mewtwo Y AND Mega Mewtwo X. Mewtwo was already incredibly powerful on its own, but both versions of Mega Mewtwo have a BST of 780, which is 60 points higher than Arceus. It doesn't help the Base Special attack (for Mewtwo Y) and physical attack (of Mewtwo X) are now the highest of all Pokémon, at a whopping 194 and 190 respectively. Looks like the claims of him being the world's strongest Pokémon have finally come full circle now.
Mega Kangaskhan's signature ability, Parental Bond, makes you use the same attack twice in the same turn — while the second attack (of the child) is half as powerful, it still amounts to a free Choice Band boost with Substitute-breaking powers on top. What really makes her terrifying are the many potential Combos you could exploit with this ability: just for two examples, Power-Up Punch, a Fighting-type attack that raises Attack, effectively gives a Swords Dance boost while attacking, while Body Slam, obtainable from Generation III, has a 51% chancenote The chance of a secondary effect at least once in the two hits is 1-(1-p)^2; this comes out to 0.51 for Body Slam's p=0.3. of paralysing the opponent. Her movepool is expansive enough that she can beat most of her "counters" with the right move. She also has a first-strike move and more bulk than what you'd normally think of as bulky, like Celebi.
Mega Lucario has been compared to "a Choice Band Terrakion with a Choice Specs Keldeo strapped to its back for special attack and Starmie for speed". This is because of its sizeable boost to all offensive stats (including speed, which is now faster than things you'd normally think of as fast, like Latios) along with the Adaptability ability, which turns the 1.5x STAB bonus into double damage. Combine this with the fact that it has ways to boost both physical and special attack, and you have something, that will raze entire teams with no support at all. To put this in perspective, a +2 Adaptability Close Combat has a chance of OHKO'ing Hippowdon and Skarmory, and a +2 Aura Sphere 2HKO's Chansey and Blissey with Special Attack, something only Kyogre could before boast. And as the icing on the broken cake, it has three forms of priority, all with effective 80 base power.
The series' original game breaker, and to this day one of the most dreaded, is Double Team. This move (and the very similar Minimize) increases the user's evasion - on the first use, all moves have 3/4 of their normal chance of connecting, the second use drops this to 3/5, and can continue until accuracy is 1/3 of the usual value. And when it was introduced, only one move could counter its usage - the underwhelming Swift. If an opponent had the chance to buff their evasion enough, or could heal on top of that (such as early top tier favorites Alakazam, Starmie, or Mewtwo), you basically had to hope to get extremely lucky if you wanted a chance to win. Even with a proliferation of ways around it (10 different moves guaranteed to hit, plus a few other ways to neutralize it), the effects of evasion are feared so much that banning the two moves guaranteed to raise it is perhaps the most common house rule called, items giving one level of increase are considered overpowered, and things that have a chance of increasing it (the move Acupressure and the ability Moody) are also sometimes called game breakers themselves. And it is ridiculously easy to get, to the point where the full list of fully evolved Pokémon that can't learn Double Team at all is as follows: Ditto, Unown, Wobbuffet.