Ditto

By virtue of its Hidden ability, Ditto makes for one of the greatest revenge killers in the game. This ability, Imposter, allows Ditto to instantly transform into the Pokemon it switches into, copying said Pokemon's moves, ability, and typing in addition to every stat except HP. This not only allows Ditto be an excellent revenge killer with a Choice Scarf, but also lets it scout movesets of unpredictable Pokemon. Ditto can also perform well on teams that have a weakness to setup sweepers because it naturally deters the opponent from setting up. Ditto also has the ability to copy an opponent's Mega Pokemon, potentially allowing the use of two Mega Pokemon on a team at once. Outside of these roles, Ditto can do very little, and as a result it is a very one-dimensional Pokemon. Additionally, Ditto's HP stat is pitifully low, something only made worse by its susceptibility to every damaging entry hazard. Ditto also struggles to perform effectively against more defensively inclined teams, making it hard to fit into a team slot.