My favorite characters were always the Big Bads, mainly because they seemed to be written and played with so much depth. On "Buffy," Mayor Wilkins was probably the most charming, witty, outgoing person that you would ever want to meet, and yet underneath all of that charm and good fellowship there was a cruel, soulless, conniving monster, drunk on power and lusting for more. I don't think for a second that he was portrayed that way and as a politician were coincidental.
On "Angel," my favorite Big Bad was the Beast, because he was exactly the opposite of Wilkins. Still evil, still powerful, but blatant about it. Powerful, destructive, ruthless, and nearly invulnerable, but, despike all of his power, totally subservient to his master.