Interesting question. You know, part of what I love most about Buffy is the way things can shift and change and evolve. Spike and Dru were definitely the "big bads" of the first half of season 2, and then Angelus came along and took up the mantle.
But look at where they ended up at the end of season two. The three main bad guys: Spike made a pact with the hero, knocked out Dru and ran away. Angeulus was restored to Angel and then immediately martyred by his true love to save the world. Compare that to the previous and following seasons (Master - killed. Mayor - killed) and you'll have a good idea of why I think season 2 is so brilliant. Of course, I think season THREE is brilliant, too (and season four, and....) but I like the unconventional way Season 2 is resolved. And on a random unrelated note, BOO HISS to Smallville ending THEIR season 2 with a copycat of Buffy's (Clark Kent running away to the big city).