IIRC the topic title is talking about the best Ghost pokemon, not the coolest.
There are multiple possible definitions of "best", and not all of them have anything to do with which is the most competitively viable. My answer to this topic would be Shedinja because I like its design the best. If the topic were about Electric Pokemon, my answer would be Ampharos.
My NIDOKING brings all the boys to the yard: http://goo.gl/ExAoe