Angel, by a country mile. I loved Buffy in the early years, but it declined sharply in season 4. The writers were in way over their heads with The Initiative, and despite their best efforts to darken the tone it came off as silly and unintentionally hilarious. Season 5 was a mild improvement, but seasons 6 and 7 were, IMO, abysmal.
In contrast, Angel went from strength to strength, with the only missteps being Connor and that stupidly convoluted arc that dominated most of season 4.
I think Connor might have been more tolerable if anybody
other than Vincent Kartheiser had the part. His looks, his voice, his line delivery...it all just annoyed the hell out of me.
But character-wise, I loved how the writers grew Cordelia and Wesley, and later Fred, but I wish we got to see more of Kate Lockley. She was a champ