Mawile

Mega Mawile is a terrifying threat in VGC. Its powerful STAB Play Rough and priority in Sucker Punch serve it well in tearing down anything in its way. It has decent bulk backed up by Intimidate, allowing it to take some hits particularly well, and its defensive typing gives it a good matchup against foes like Mega Kangaskhan, most Xerneas, and most Dragons, which are all very common in VGC. Its low Speed allows it to take advantage of the effects of Trick Room very well. These things make Mega Mawile a very reliable asset to many teams. However, Mega Mawile can't hit threats such as Primal Groudon and Ferrothorn for much damage, and those will quickly punish Mega Mawile for it. It can't take hits from most of the restricted legendaries well, especially special attacks; Primal Kyogre gets the OHKO at all times and Xerneas can do more than 70% with Moonblast after a Geomancy boost. These things definitely hold Mawile back, but they are heavily outweighed by its very unique matchups and brute force.