Yanmega

Yanmega has an excellent Special Attack stat, very respectable Speed, and two of the most powerful abilities in the game at his disposal. With Tinted Lens, Yanmega is capable of 2HKOing even dedicated walls who resist his STAB attacks, an impressive feat. Meanwhile, if he instead chooses to use Speed Boost, he will outspeed the entire metagame, including Choice Scarf users, after a turn or two.

It's hard to imagine Yanmega being anything other than top-tier OU after considering his strengths, yet the fact remains that he's not. Yanmega suffers from an utterly crippling 4x weakness to Stealth Rock that has completely nullified his presence as a standard of the metagame. Weaknesses to common Fire-, Ice-, and Electric-type attacks aren't helping Yanmega's position either. These weaknesses, coupled with Yanmega's not-so-stellar defenses, have sealed the dragonfly's fate and unfortunately banished him to the bottom of OU.