If the Trio gets a spot on the list, Spike, Dru, Faith, Caleb, and I guess the Anointed One deserve spots on the list as well.
The Mayor > Angelus > Spike >> Glory > Caleb > Drusilla > The Master > Faith >> Dark Willow > The First > The Trio > Adam > Annoying One
EDIT: Just got an idea, how about the Angel villains?
Wolfram and Hart > Jasmine > Hamilton > Holtz > Darla
Maybe Hamilton's cheating a little. Not sure where they'd rank if I combined the lists, I'd need to think about it.
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xfd dude, it's just a popularity contest on the internets.
-Vlado