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.
Well back in Gen 1 I wanted to use Ghost Types, but the main thing Ghosts were used for (Countering psychics) was pointless because all three ghosts were part of one evolutionary line, and that line was half poison.
Why does psychic's only real weakness have a weakness to psychic?
And it wasn't till years later I found out about the Ghost Immumity glitch..
So when Misdreavus came along in Gen 2 it felt.. cathartic to have what I saw as a "real" ghost.