It's a lame answer but its Haunter. It's been my favorite since it was my main pokemon way back in the first pokemon game I ever played (I had no one to trade with to evolve to Gengar & one of my siblings released my starter)

I really like ghost and poison types in general so those are what I usually stick with. Some runners up would be Muk, Golisopid, Pallosand, and Dhelmise.