Woo, let's give this a shot.
Serperior
Though a rare sight in BW OU, Serperior is a common menace in Dream World due to its ability, Contrary. With Contrary reversing the drawback of Leaf Storm—a sharp decrease in Special Attack—Serperior is able to grow more and more powerful while repeatedly firing off high-powered attacks, easily nabbing momentum; while its coverage is limited mainly to Leaf Storm and Hidden Power, a neutral hit from either move will still pack a punch once Serperior reaches a couple boosts. Furthermore, Serperior possesses a plethora of support options, such as dual screens, Leech Seed, Glare, and Substitute, all of which can give the opponent a hard time. Once it has a few turns to repeatedly use Leaf Storm, Serperior becomes a gigantic threat; however, dispatching it with several strong blows as soon as possible will preclude this from happening.