Angel is to Buffy as DS9 is to TNG, in a lot of ways. And seeing how DS9 is my favourite Trek, I agree with you. But not by too much, mostly due to Angel's S3 (especially) and S4, which I thought were quite dissapointing after the total brilliance of S2. I haven't watched the second half of S5 yet, that's where I'm currently at in my first time run through the Buffyverse.
S6 is my favourite season of Buffy, though, despite all it's flaws. And posibly my second favourite Buffyverse season, after AtS S2. Go figure.
Probably because I think it's the darkest and least formulaic season of Buffy.