Mewtwo

Feast your eyes on the most destructive Pokemon in any tier in history. Not one stat Mewtwo has can be called bad or even average, including the highest Special in the game and a tie with Aerodactyl and Jolteon for second-highest Speed. It's not just an offensive juggernaut: Mewtwo's bulk is absolutely titanic, stomaching even blows from the mighty Snorlax and Tauros. Its Psychic typing comes into this as well, giving it effectively no type-based weaknesses to exploit. Because of these qualities, taking Mewtwo down often requires a concentrated effort from multiple Pokemon. As if that wasn't enough, it has a plethora of fantastic moves, such as Amnesia, Recover, STAB Psychic, Self-Destruct, and more. If given even a single turn of breathing room, Mewtwo can immediately take over a game. There is no true way to defeat Mewtwo head-on other than sending in your own, statusing it, or getting plain lucky. Thus, many RBY Ubers games are decided by a Mewtwo mirror match. Mewtwo was designed to be the world's strongest Pokemon, and Game Freak certainly succeeded: Mewtwo has zero inarguable flaws and is an automatic pick on any RBY Ubers team that aims to win.

Mewtwo's only arguable flaws stem from it being quite literally too powerful for a Game Boy to process. A maximum DV and EV Mewtwo can end up overflowing its Special if it's boosted to +4 or higher and subsequently sustains a Special drop, minimizing its Special and leaving it effortless to KO. Similarly, Mewtwo's Speed gives it a very high critical hit rate—25.39%—which, combined with the fact that critical hits ignore the attacker's offensive boosts, means that +3 or higher Mewtwo will sometimes deal less damage than expected. Finally, while Mewtwo is incredible, it will always have some check it has trouble beating outside of brute force, as it often only has two moveslots available after Amnesia and Recover. However, brute force remains a fine option in a tight spot because Mewtwo is an utter juggernaut.