You know, I've always liked Angelus, he's always been my favorite Buffy/Angel villain (by quite a bit actually, even including Wolfram & Hart), but I've never really thought of him as one of the best ever for whatever reason.
But you know, he is. Totally. When I think of my favorite shows, Buffy, Angel, Smallville, LOST, Las Vegas, The Wire, Veronica Mars, Game of Thrones, and when I look at all the villains they have come out of those great shows, none of them stand up with Angelus. His arcs in S2 of Buffy and S4 of Angel are just so incredible. Home run grand slams, both times.
He's the single most threatening villain I've ever seen, bar none. He gets inside the heads of the heroes so much that they basically have no way to deal with him. I mean, S4 of Angel was just awesome in that regard. Wes, Gunn and the rest totally underestimate Angelus and within half an episode he's already basically taken them all out.
Angel is one of my favorite heroes of all time, and Angelus is THE best villain of all time.