I've watched Buffy through twice and Angel once. I like Buffy more overall, though Angel had flashes of dark brilliance. It was just overshadowed in the long run by goofy plots that manage to out-silly Buffy (see: traveling to demon dimensions in a convertible, end of season 4, almost everything that went on in Wolfram & Hart season 5, etc.).

My favorite character is definitely Spike. My favorite episodes are the ones with Spike in them (especially: Lover's Walk season 3, the one where Spike/Buffy get "married" in season 4, and more serious ones like Lies My Parents Told Me in season 7). Angel season 5 kinda pissed me off because it largely ignored all of Spike's development and maturity in season 7 of Buffy to turn him into a goofy sidekick (which part of me was ok with, as goofy Spike was hilarious in seasons 3-6 of Buffy).

As far as Big Bads go, it's tough to beat Angelus, in either Buffy or Angel. I thought the Beast/Angelus subplot was the best part of season 4 Angel, before the show ruined both it and Cordelia. Dark Willow was also great.