Crobat and, more recently, Drapion...
I've always loved poison-types, as their battle tactics seem to match the sadistic slow and agonizing death strategy used by rogues and thieves in other RPGs, and Crobat emerged as my favorite early on because of his sheer speed making him extremely evasive after a couple of Double Teams while the opponent is slowly dying from Toxic and a confusion spell. Drapion has that same essential appeal, but is more defense-oriented than speed-oriented.
Honorable mentions: Haunter, Sandslash, Venomoth, Ariados, Misdreavus, Forretress, Dustox, Sableye, Shedinja, Bronzong, Toxicroak. :P