The question should really be “Who’s the second-best Buffy Big-Bad, since all good and righteous people know that Glory is the correct answer” 
One note: It’s not “What’s the best season?” (I might argue S3 or S4), it’s only about who was the best/most/fun/most effective Big Bad? The one you’d like to see come back if they ever do a “Buffy Reunion” thing.
Just to remind everyone:
S1: The Master-Ugly Ass vampire who read the Villian’s Handbook and decided to do everything wrong anyway. He was kind of awesome in that he gloated over fallen enemies, used elaborate death-traps, etc.
S2: Angelus: Angel gone stupid and evil. Like the Master with no panache or impulse control. (For example, he killed Miss Calander, rather than vamping her which would have been much worse)
S3: The Mayor: "All I want is to be a big snake. Is that so wrong?
S4: Adam the silly-looking man-child demon/robot/clone
S5: Glory. What more need be said? She kicked Buffy’s ass and almost won, PLUS she looked great in a bubble bath.
S6a: The Terrible Trio–The three nerds
S6b: Dark Willow (I’ve never figured out which was the actual big-bad)
S7-The First Evi (which I never really got.
So–big bad only, season quality notwithstanding, who do you vote for?
(Poll to follow)