Isn't it obvious?
No it isn't.
Zubat family is my favorite. I know many trainers find them annoying (and yes, I have seen quite many posts regarding Zubat's abundance in caves and such) but pity on the lil' guy will ya? He's blind. anyway, I like it simple because of their pure badassness. And when it evolves to four-winged crobat. OSOM.
Gastly line comes up next, simple because of Gengar's pure malice. (and coolness)