Buffy, easily. Both were great shows, but Angel just lacked the thrall that Buffy had. I think the main thing I didn't like about Angel was that, aside from season 5, every season had a streak of really boring episodes based around the main arc, but there would be some really spectacular ones at the end. Plus, overall, it just wasn't quite as good...the plots weren't as interesting, the writing was stale at times, and, most importantly, the characters weren't as compelling. By the end of Buffy, I actually felt a personal connection to the main characters, but I can't say the same about Angel (as much as I loved a lot of the characters).
In my opinion, the only thing Angel had over Buffy was the visuals. They had better stunts, special effects, sets, et cetera, but they didn't compensate for the things that the show was lacking.