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.
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.