Hydreigon

Hydreigon's one of the hardest hitting Pokemon in the ruleset. Its Choice Specs Draco Meteor hits like a nuclear bomb, OHKOing most of the Pokemon in the format. To back its Draco Meteor up, Hydreigon also has a powerful secondary Dark-type STAB, a wide movepool to cover gaps in its coverage, decent defenses thanks to high base stats, a few useful resistances, and Levitate. However, Hydreigon's 4x weakness to Fairy in a format where Sylveon is very common is a huge setback. A Hyper Voice from even the most defensive of Sylveon will cleanly OHKO Hydreigon. There's also an array of Steel-types that can sponge Draco Meteors and a whole host of faster Ice-, Fighting-, and Dragon-type Pokemon to keep it in check.