Gengar

By virtue of its high Special Attack and Speed stats, Gengar is an exceptional wallbreaker in the OU metagame. It can take advantage of the tier's general lack of Ghost-resistant Pokemon to fire off powerful STAB Shadow Balls while also pressuring the tier's myriad of Fairy-types thanks to its Poison typing. Gengar also has a decent amount of coverage and utility options at its disposal, including useful moves in Focus Blast, Thunderbolt, and Taunt. Unfortunately, Gengar is horribly frail and thus easily folds to faster Pokemon, Choice Scarf users, and priority, which are all quite common in OU. It also has trouble taking strong neutral hits because of this, and at times it over relies on KOing its foes as a result. Gengar's fragility can also limit its opportunities to switch in, which can prevent it from consistently wallbreaking. Lastly, Gengar is quite weak to Pursuit from the likes of Weavile and Tyranitar, which requires that it is played carefully in order to prevent it from being trapped and KOed.