With the release of Pokémon Scarlet & Violet, a new incredible pair of powerful legendary Pokémon, Koraidon and Miraidon, have been released. Every generation has overpowered Pokémon that are typically banned from singles and some doubles formats, and gen nine is no different.
The most powerful Pokémon aren't always "box art" legendaries, but those are at an advantage. While Pokémon in lower tiers are often defined by having an interesting niche, the strongest additions in the history of the game dominate on every level. They have fantastic moves, stats, abilities, and multiple roles, and centralized the competitive game around them in their generation and were often strong in future generations as well.
Gen 1: Mewtwo
In a generation where the majority of Pokémon were the standard "elemental animal" design, Mewtwo's alien appearance made it one of the strangest looking Pokémon in the first generation. However, it was also the strongest.
Mewtwo was the Pokémon that the Uber tier on the battle simulator Pokémon Showdown was invented for. There weren't any Pokémon in the generation that could come close to standing up to Mewtwo. It was so powerful that any matchup where it was allowed would almost certainly come down to a Mewtwo vs Mewtwo mirror match since it so effortlessly defeated every single Pokémon in the first generation, making it by far the most powerful Pokémon of all time for its own generation.
Gen 2: Ho-Oh
Generation 2 brought along with it the concept of box art legendaries. Suddenly, each generation brought two incredibly powerful Pokémon along with it. In the case of Gold & Silver, it was Lugia and Ho-Oh.
Ho-Oh is one of the most used Pokémon in the game during all second-generation formats it was allowed in, but what truly put it above Lugia was its versatility. The lack of Stealth Rock in the second generation of Pokémon was part of what made Ho-Oh such a menace, letting it switch into weaker Pokémon with no fear and obliterating them with its ridiculously overpowered signature move, Sacred Fire.
Gen 3: Kyogre
Ruby & Sapphire introduced abilities to the world of Pokémon. Along with the introduction of abilities came the two clear favorites for most overpowered ability in the game: Groudon's Drought and Kyogre's Drizzle.
Drizzle ended up defining the competitive aspect of Pokémon for the next decade, and to this day, Drizzle and Swift Swim have combined to be the backbone of a powerful archetype known as "rain teams." The most dominant user of Drizzle has always been Kyogre. Its outrageous stats and rain-boosted Water Spout have made it possibly the most consistently dominant Pokémon in the history of the game throughout the ages.
Gen 4: Arceus
Generation Four brought the god of all Pokémon, Arceus, to the game. Its ability, Multitype, which changed its typing based on the plate that it was holding, allowed it to be 17 very different Pokémon depending on the type.
Arceus was by far the most versatile Pokémon of all time, but its power on the competitive side of Generation Four was sadly mostly theoretical. In simulated formats where Arceus was allowed to use its true potential, it was arguable that the 17 Arceus forms were probably all in the top 20 Pokémon available. However, since there was never a legal version of Arceus that players were able to train and use like a normal Pokémon, its immense power was purely theoretical.
Gen 5: Thundurus
Generation Five was when the competitive scene in Pokémon really started to gain a larger following, and VGC became one of the newest alternative ways to enjoy Pokémon. One of the kings of VGC during this time was Thundurus, the strongest of the three "genie" legendaries.
Thundurus's Prankster ability was part of what made it so brutally powerful, as it made all of its non-attacking moves first. This made Thundurus a great partner for rain teams to give a priority Rain Dance to help with weather wars, but its ultimate skill was priority Thunder Wave to cripple teams completely. On top of its utility, it hit hard and had fantastic typing in Electric/Flying.
Gen 6: Xerneas
Pokémon X & Y brought the Y-shaped legendary Yveltal and the X-shaped legendary Xerneas. While Scarlet & Violet has already changed the competitive game greatly, X & Y's additions of Mega Evolutions changed the game drastically.
In a generation filled with powerful new Mega Evolutions, Xerneas still single-handedly defined the metagame without one in both singles and doubles. Thanks to its signature move, Geomancy, Xerneas could double its Special Attack, Special Defense, and Speed in just one turn with help from the Power Herb item. This made it a dominant threat that could completely sweep through teams effortlessly with its Moonblasts. It wasn't even countered by Steel and Poison types due to Hidden Power Ground coming off a doubled Special Attack stat, making Xerneas almost without counters.
Gen 7: Incineroar
Most generations were defined by incredibly powerful threats that dominated the game with absurd stats. However, Generation Seven taught players that there's more than one way to be overpowered, thanks to the most game-defining utility Pokémon of all time, Incineroar.
Incineroar became the king of doubles, blessed with one of the best doubles abilities in Intimidate and several of the best utility moves in the game. Its combination of Fake Out and pivot moves (including the gift of Parting Shot in gen 8) with its Intimidate ability let it bounce in and out of battle, absolutely crippling Pokémon in its path. It wasn't only made for utility, either. Incineroar could hit hard with super-effective Flare Blitzes and Throat Chops. Incineroar was so dominant for two generations that some Pokémon were only usable due to being paired with Incineraor's utility.
Gen 8: Zacian-Crowned
While the first generation had the most dominant Pokémon of all time in Mewtwo, the eighth had the most broken Pokémon of all time, Zacian-Crowned, the undisputed strongest Pokémon in both singles and doubles.
Zacian-Crowned came with the best typing in the history of Pokémon, which is Steel/Fairy, along with the highest Attack stat in the entire game when its ability, Intrepid Sword, was factored in. Zacian was so overpowered that some players at the highest level of play brought in the meager Quagsire, whose Unaware ability let it be one of the few Pokémon that could take a hit from Zacian's boosted attacks. Its dominance was so severe that singles players banned it even from Ubers, a tier created for Pokémon that were already too strong to use normally.
Gen 9: Miraidon
Miraidon was always going to be a heavy contender for Scarlet & Violet's strongest Pokémon, but after the stats and ability of the Pokémon were revealed to fans, it was clear that Miraidon was going to be a monstrous threat.
With an ability in Hadron Engine that boosts Miraidon's Special Attack, Electric moves, and its allies with the Quark Drive ability, it's all but destined that Miraidon will be the dominant force in both VGC and singles formats where it's allowed. Unlike Koraidon, whose ability doesn't boost any of its own STAB moves, Miraidon will be hitting like a truck while also boosting its teammates in battle, all while boasting a ridiculous speed stat.
Source:gamerant.com