Ditto

Ditto has a very unique ability in Imposter, which allows it to copy any Pokemon, stat boosts and all, making it one of the best revenge killers in the metagame. Ditto is capable of turning games around by preventing opponents from winning with most setup sweepers. Ditto also works extremely well with the move Swagger, copying the +2 Attack boost from opposing physical attackers; it copies all the boosts from the foe but none of the status the opposing Pokemon may have. This makes running Swagger much less risky and a legitimate strategy. However, Ditto isn't always good against certain teams, especially defensive ones, which don't have many Pokemon that Ditto will benefit from copying. It also needs to paralyze the foe if it's not running Choice Scarf to avoid Speed tying with it, so it's not always reliable against Ground-types, such as Garchomp and Mamoswine. Ditto also has a very poor HP stat; as this is the only stat Ditto doesn't copy from the foe, it's usually very frail.