Regice

Regice boasts one of the highest Special Defense stats in the game, which allows it to beat many special attackers 1v1, such as Chatot and Rotom-F. It's also helped by its high Special Attack stat and a solid movepool consisting of moves such as Thunderbolt, Ice Beam, and Focus Blast. Regice's low Speed is an issue, but it has access to both Rock Polish and Thunder Wave to alleviate this, allowing it to function as a potent sweeper or a bulky tank. Ice typing gives it a decently powerful STAB move to work with in Ice Beam, but it isn't a great defensive typing and hampers Regice's ability to switch in and out due to a Stealth Rock weakness. Furthermore, Regice's recovery is limited to Leftovers, meaning it will often be worn down quickly. Its weaknesses to Fighting, Rock, Steel, and Fire make its superb defenses less effective. However, Regice can run through teams by setting up a Rock Polish and taking advantage of its amazing coverage to become a serious threat later in the game.