I'd have to say Buffy Season 3 although Buffy Season 2 comes very close.
I love those two seasons the best because there so Buffy/Angel centered and I'm a big bangel fan!! I prefer Season 3 better than Season 2 because a lot more happened. The Bangel relationship had moved from teen romance to that bitter sweet, heartbreaking stage, there was the introduction of Faith (the whole bad slayer concept was excellent), Willow was introduced as a witch and the relationship between Buffy and Giles was really focused on, not to mention the fight scenes were great to watch, especially the fight between Buffy and Faith in Graduation.