Season 6 was terrible save for a few episodes. Magic addiction, ruining Anya, Giles gone, Buffy emo...blugh. Season 5 was the best post-high school season by far, sans Dawn. My rankings:
3 > 2 > 5 > 1 > 7 > 4 > 6
7 had Conversations With Dead People, which I think is the best episode of Buffy since The Body. Plus the finale was mostly excellent.