Dragonite
Dragonite is a very powerful pseudo legendary Dragon/Flying type Pokémon. It's basically the Pokémon equivalent of the noob tube. It has an extremely OP ability in Multiscale, which cuts damage from the first attack it takes while at full health in half. This can be circumvented with the use of Stealth Rocks to chip a god 1/4 of its HP away when it enters the battle field, but that rarely ever matters since it's commonly used as a lead and almost always is accompanied by a Pokémon capable of getting rid of said entry hazard. Because of said ability, even most Ice type attacks (which it has a 4 times weaknesses to) won't OHKO a standard Dragonite, much less a bulky set. It's also problematic in that it has access to Dragon Dance, which raises both Attack and Speed one stage, as well as Extreme Speed, a move with high priority that almost always attacks first. Its Outrage attack is a death sentence, especially after an Attack boost. You would think the existence of Fairy type Pokémon would hinder its performance in today's metagame due to their immunity to its Dragon type moves, but such is sadly not the case because Dragonite also has Steel Wing and Iron Tail in its arsenal to one shot any pesky fae in its way. Dragonite is truly the king of all OU Dragon type Pokémon.
Guy 1: So I ALMOST had him, but then he pulled out his Dragonite and swept my team after getting a Dragon Dance up while I switched into my Fairy type and wrecked everything I had left.
Guy 2: Don't feel bad dude, it's fucking Dragonite. Son of a bitch has been OP since Gen V.
Guy 1: Ugh, why the hell doesn't smogon just ban the damn thing?
Guy 2: For real. It's too easy to sweep with and there aren't that many good counters or checks against it. It's just a pain in the ass.