Swampert

Mega Swampert possesses one of the best defensive typings in the game in Water / Ground, with resistances to Fire, Rock, and Steel and a critical immunity to Electric-type moves, the bane of most Water-type Pokemon. This great defensive typing is further fortified by Mega Swampert's amazing bulk, allowing it to survive a hit from powerful attackers like Porygon-Z, Dragonite, and Pheromosa and put them to sleep with the combination of Yawn + Protect. Mega Swampert's Ground / Ice coverage in Earthquake and Ice Beam hits many common Fire-, Steel-, Dragon-, and Ground-type Pokemon super effectively like Victini, Magnezone, Dragonite, and Garchomp. However, Mega Swampert's main selling point in Yawn is also its biggest downfall, as its reliance on sleep turns makes it a shaky check to Pokemon that can 2HKO it like Dragonite, Mega Medicham, and Mega Lopunny, relying on the 66% chance that opposing Pokemon stay asleep for at least two turns. Furthermore, with a lack of usable stat-boosting moves and an over reliance on sleep turns, Mega Swampert is usually countered by a plethora of stall-oriented Pokemon with access to Taunt like Deoxys-S and Mega Sableye or Defense-boosting moves like Mega Slowbro and Mega Altaria. Lastly, Mega Swampert's Water / Ground typing gives it a glaring 4x Grass-type weakness, making it susceptible to Grass-type Pokemon like Serperior and Grass-type coverage like Solar Beam from Mega Charizard Y and Grass Knot from Greninja.