Ditto

Ditto has always been deemed a gimmick, ever since its RBY days. The same went for the start of Generation V, where the same old Ditto was never competitively viable. Then came the shift to BW2, and with it came a new ability for Ditto, a saving grace that would bring it from the lowest depths of NU: Imposter. Imposter transforms Ditto upon switch in, allowing it to avoid wasting a turn using Transform. This ability is what launched Ditto from a useless gimmick into a ruthless revenge killer. Ditto is a setup sweeper's worst nightmare, and when equipped with a Choice Scarf, it can outspeed and kill almost everything in UU, provided it can switch in with minimal damage.