Serperior

No one would have ever thought about using Serperior until the long-awaited release of its Dream World ability, Contrary. This ability, when combined with Leaf Storm, gives Serperior a 130 Base Power STAB move with the side effect of a Nasty Plot, allowing it to attack and boost its Special Attack at the same time! This would have turned Serperior into a terrifying special sweeper if it weren't for its middling special movepool, which is limited to Dragon Pulse and Hidden Power for coverage. This has led to the rise of defensive Serperior being the more popular set as a way to check physical attackers with the combination of Reflect, Synthesis, and Rocky Helmet, with Leaf Storm tacked on to maintain an offensive presence. Serperior can also make use of several other great support moves including Glare, Taunt, Light Screen, and Leech Seed.

Serperior is gifted with a great Speed stat, which allows it to outspeed key physical threats such as Garchomp and Mega Kangaskhan, as well as anything slower, meaning it can quickly set up Reflect before being attacked or pick off a weakened foe with Leaf Storm while gaining the Special Attack boost. Being weak to common Pokemon such as Talonflame, Blaziken, Mega Salamence, and Greninja, as well as being a little frail without Reflect set up and weak before a Leaf Storm boost, might make it seem as though Serperior would be a poor choice for your team; however, as far as Grass-type options go, having access to Contrary and not taking up a Mega slot are huge points in Serperior's favor.