Season 3. (To me, best to worst is 3, 2, 5, 6, 4, 7, 1)
I mean, I could explain why I love Season 3 with one word: Faith. I dearly love Faith, but that's kind of selling the season short. There are very few bad episodes in season 3 (despite how good season 2 was, half of the episodes were bad), and I like the storyline more overall (really, I see season 3 as Faith's story, not Buffy's).
What's really interesting to me is that, first time through, I thought season 2 was better than season 3, but looking back at it in hindsight, season 3 is better. I wonder if anyone else has felt the same way and why that's the case.