Season 1 - a lot of terrible stand-alone episodes (demon fight club, christ) with some bright spots
Season 2 - relatively interesting but often silly and pretentious overarching plot that suddenly ends and turns into goofy fantasy
Season 3 - middling pregnancy drama --> awful romantic comedy ---> actually well-written and suspenseful last 6 or so episodes
Season 4 - writers obviously just making shit up as they go along but every episode ends in a wildly different place than it started, I remember having a blast watching it at the time.
Season 5 - half-successful reboot, Whedon tries to write an operatic tragedy which is pretty annoying but the finale is solid.
I thought Boreanaz's best moments on Buffy were when people were making fun of him, or he was inadvertently looking dumb. And I think the occasional broad comedy bits on Angel don't always work, but I like that he evolves into this kind of out-of-touch dork, and his attempts in season 2 at being dark and intense are just kind of stupid.