The most powerful Ghost type Pokemon honestly,I'm giving it to the deadly Gengar. It's insane Sp. Attk and speed is a deadly combo. Also, it can use sneaky moves and combos like Toxic+Mean Look or something like Destiny Bond. And all the moves it can learn- Thunderbolt, Shadow Ball, this thing is one serious beast!
G to da E to da N to da G to da A to da R, Gengar. Yep that's right, Gengar has an insane sp. atk and high speed to make it deadly. Once you teach it Mean Look, Toxic/Hypnosis, Dream Eater/Shadow Ball, Destony Bond it's game over.