Ditto

Due to Imposter, which allows Ditto to transform into the foe and copy its stat boosts, it is one of the metagame's most potent revenge killers when equipped with a Choice Scarf. In addition to revenge killing, Ditto's ability allows it to scout the entire movesets of opposing Pokemon without them even revealing their moves, meaning that it is effectively the best scout in the game. While Ditto may not always get the opportunity to sweep the opponent's team with the team's own setup sweeper, its mere presence on the field is often enough to dissuade the foe from setting up, greatly limiting the effectiveness of their sweepers. However, due to its terrible HP stat, Ditto has a very hard time switching in, meaning that it must switch in either using pivoting moves such as Volt Switch and U-turn or when a teammate has fainted. Ditto also faces competition as a revenge killer from Fake Out + Extreme Speed users such as Stoutland and Ursaring, which, while not as effective as Ditto at revenge killing, aren't as one-dimensional and can accomplish other things outside that role. In addition, because of its Choice Scarf, Ditto cannot defeat defensive sweepers that cannot damage themselves sufficiently, such as Mega Gyarados, Mega Slowbro, and Clefable. Finally, Ditto's usefulness is largely dependent on the opponent's team, and it will rarely be able to do much to teams that are reliant on bulk as opposed to attacking power due to its Choice Scarf and low HP limiting its ability to function as a defensive Pokemon that it transforms into.