Ditto

Ditto has been doing the same thing since its inception: transforming into its opponent. With Imposter, Ditto is able to copy the opposing Pokemon and its stat boosts almost flawlessly without wasting a turn setting up, allowing it to revenge kill setup sweepers, such as Shell Smash Gorebyss, Fraxure, Sawsbuck, and Shell Smash Carracosta. With a Choice Scarf attached, Ditto is guaranteed to outspeed those same offensive threats. Unfortunately, Ditto cannot copy the HP stat of its opponent, making it frailer than its target in most situations, as Ditto's fully invested HP stat is only equivalent to an uninvested base 79 HP stat. As it must carry a Choice Scarf, Ditto will always be limited to one move. As a result, Ditto has difficulty beating most setup sweepers, including Bulk Up Gurdurr and Braviary; and Swords Dance Samurott; and Calm Mind Serperior and Musharna, making it an inferior revenge-killer, the one job it can actually do. This also makes it difficult for Ditto to pull its weight against defensively oriented teams, as its power is in the hands of the opponent and the opponent can often plow through it. While Ditto can work, it's far too situational to fit on most teams, as there are too many revenge killers in NU that can do its job more reliably.