Federation didn't work properly for the original post - this is a repost.
My favorite is Drifblim. I remember how mysterious it felt having Drifloon be only catchable on Fridays in the original Diamond and Pearl versions. It made breeding for their eggs more exciting than the regular kind of Pokémon. Also it’s a living balloon which I like.
Gengar is the obvious choice right? The whole evolutionary trio in the Pokémon tower in the anime had a lot of character for appearing in all of one episode
Banette! I love the lore, the look, and the merchandise.
But really I love all ghost Pokémon and choosing is hard. Here's my cube wall as proof:
Haunter! Gengar is the evolution, but they go from an asshole trickster to a malicious dick on evolution. When I play, I like to pick Pokémon I haven't had in my party before, with the one exception of Haunter, because they're that great.
Aegislash is likely my favorite one. It has the best design and the most unique ability of you ask me.