Gengar!

I wish he'd be a pure Ghost type so much. The Poison typing didn't ever make sense to me, and makes him quite useless in this game.

I have still yet to find a use for Gengar, as he has very limited type advantages and quite a few bad matchups

It does not need type advantage, it has STAB, great movesets and a high att-stat. Just try it against anybody except blissey/Snorlax, dragonite works very well, for example.