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.
Aegislash is likely my favorite one. It has the best design and the most unique ability of you ask me.