I happen to be watching some Buffy lately, and just watched that scene in Season 7 where The First cycles through all the villains of previous seasons while tormenting Spike.
I would have to say The Mayor of season three is my favorite. So gleeful, so goofy, yet dark as hell when Faith is threatened.
I’ve gotta agree with the mayor, with Glory in a not-that-close second. I think the mayor was awesome because something about a charming politician being evil made him the most realistic and sinister of the villains going, evil demon hell stuff notwithstanding. Plus he was hilarious, which wasn’t really true of the other villains - he was the best character, while some of the other big bads weren’t really even characters at all.
Even though I hated the season and the big bad, though, Season 4 gets my vote as best last episode.
Glory. The insanity, the power, the clothes.
2. Angelus
3. The Mayor
4. Warren
5. The Master
6. The First
7. Adam
The Mayor is gonna blow everyone else out of the water.
Angelus, Glory, and the Master will probably get a look in, but…the Mayor, man, the Mayor.
The Mayor - “And boys…watch the language.” I just like the mayor. He’s a nice guy.
Angelus - My only problem with him is that I do not buy Angelus would want the world to end. Otherwise, he’s amazingly cool and evil.
Glory
The Master
Warren - Again, I kind of like Warren.
The First - Caleb is fun, though. The First evil is kind of boring, though. Whatever. I like the 7th season, though.
Adam - Hard to believe, but this is one of my favorite seasons of Buffy despite it’s villain. Adam is just dull.
Crap! I hit the wrong button and can’t unclick it! Stoopid clicky dots.
I meant the Mayor, of course. Than Angelus, The First, Warren, The Master, Glory and Adam, in that order.
1.) Glory. The only BB that made me feel scared. Also, best season finale ever.
2.) The Mayor. Very close behind Glory. Really a tie for first.
3.) Adam. Not so much him, but he instigated a brutal season finale battle, number 2 behind S5. Also I like the fusion of magic and technology.
4.) the first. Scary, but the show had gotten old at this time.
5.) The Master. Not a bad villain, but was eclipsed by later, better villains.
6.)Angelus. A little too much teen angst.
7.) Warren. Are you kidding me? Dark Willow was the real villain here.
The Mayor, OF COURSE, then Glory & Dark Willow
I can’t decide between Angelus and the Mayor. Obviously the Mayor was AWESOME but for sheer psychological heart-ripped-out horror, Angelus is tops.
I’m surprised Glory is getting so much love. Her schtick got old fast for me.
The Mayor, hands down. Then Angelus and Adam. Dark Willow loses points for getting beaten by a yellow crayon.
The Mayor, easily. I love the story around him being a father figure for Faith and her descent into darkness.
Angelus. I remember watching the Season 2 finale and immediately rewinding and watching it again. Mayor would be second place for me.
You gotta give Warren points for eventually being genre-savvy enough to just grab a fucking gun and start shooting people. I know he’s not really “big bad” material, but I liked the concept that a miserable idiot nobody could cause so much misery. Because in real life most suffering isn’t caused by grand villains, it’s caused by pathetic assholes.
The Mayor. No doubt about it.
I guess if the question was rephrased: as in, “The most villianous Big Bad,” or “Most dreaded”, or somesuch, that Angelus would have won in a cakewalk. The Mayor may have been the most engaging and entertaining villian, but Angelus worked best from a purely dramatic standpoint-plus The Mayor’s final plan-become a huge snake, beeg deal-paled in comparison to what Angelus did (switch from Buffy’s dearest ally to most bitter enemy, and tried to destroy the world in the process).
In any event, if forced to rank them:
Angelus
(small gap)
Mayor
Faith (strange that she wasn’t listed)
Evil Willow
(moderate gap)
First Evil
(moderate gap)
The Master (too stereotypical)
Glory (too annoying)
(Huge Gap)
Adam
The Trio (these last two for vastly different reasons-I was hoping Adam would do a HeelFaceTurn at some point and rediscover his humanity, but Joss wasted that opportunity, while the Trio was hopeless from the start).
Noted for ironic poster name vs. content. 
The Judge? Oh, actual plotline Big Bad. The Mayor, yeah.
The Mayor in a walk.
“Violent outbursts like that, in front of the children? You know, Mr. Giles, they look to you to see how to behave.”
People actually liked Glory? Who knew?
The Mayor, of course, (the show was never as good after), but Angelus was a lot of fun.