Without a doubt Angelus, because he did something that no other Big Bad did, except, of course, Twilight from Season 8, but that's a totally different subject. For the first time, Buffy faced an opponent who could make the fight personal. This wasn't some mysterious nameless menace with no shared past, this was the man who Buffy loved, the man who was her first lover, and the first man who ever gave Buffy hopes for a shared future.
Okay, that was Angel, not Angelus, but Angelus exploited it at every opportuniy.