As much a Buffy fan that I am, I think Angel actually ultimately surpassed it for complexity, drama and satisfaction.
A much darker and more adult show than Buffy, it follows Angel's move to L.A at the end of season 3 of Buffy. A couple of characters followed him, and this finally allowed the terrific Charisma Carpenter to get into the limelight as Cordelia Chase and become an integral part of the story.
My favourite seasons are 2 (with the return of Drusilla and Darla), 3 (as the story of Darla and Angel's newborn child takes everyone down a very dark path) and 4 (with the longer term return of Angel's dark side Angelus and some big twists along the way for several major characters) and I still love the way the show ended after season 5, even though at the time it wasn't well recieved.
Cracking show!!