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