Regardless of whether or not Boreanaz is a good actor (or "re-actor"), Angelus is a great villain because he's not motivated by power or revenge or even bloodlust. His whole thing is emotional torture, and he's really good at it.

You see him screwing with the Scoobs all through the back half of s2, only killing when it's for maximum emotional damage. And, just to amuse himself, he messes with Spike because... well, because it's so easy. (He's already in Dru's head.)

When he comes back in Angel s4, he's been diluted a bit, but "Soulless", at least, is prime Angelus. Wes, Gunn and Fred are no match for him when it comes to head games.