Ditto

Ditto, formerly nothing but the whore of the Pokemon world, has finally risen to some degree of usefulness. While previously Ditto's only real use was breeding to obtain Pokemon with perfect IVs, it has received a tremendous boon in its new ability, Imposter. Imposter allows Ditto to instantly Transform into the Pokemon it is facing upon switching in, perfectly copying said Pokemon's moves and stat boosts, along with every stat other than HP. This makes Ditto a decent revenge killer in the tier, with its ability to check most boosted Pokemon with ease by copying the opponent's boosts and turning sweeps completely around. However, Ditto cannot be played recklessly due to retaining its poor HP stat upon transformation, and is forced to run a Choice Scarf in order to guarantee that it outspeeds the Pokemon it transforms into. Ditto is also extremely predictable as well as borderline useless against most defensively oriented teams that lack setup sweepers for Ditto to copy. Finally, Ditto suffers from RU's relative lack in setup sweepers, and there are also a few that sweepers that are incapable of doing very much damage against themselves or often utilize a Substitute, which blocks Imposter, such as Klinklang. Regardless, it is a completely viable Pokemon that is nearly unparalleled in its ability to fix up holes on teams suffering against some of RU's powerful boosting sweepers.