This is such a difficult question because each season is marked with both utterly fantastic and mind-numbingly bad episodes. But I tend to view the seasons from a distant standpoint, that is to say, the story arc for the season.
Season One was merely good. Overall it was filled with what I like to call "Monster-of-the-Week" episodes--so we don't get a real good sense of the characters in this season; they don't grow quite as much as they do in future seasons.
Season Two, on the other hand, witnesses a number of changes within the characters. Apart from the various relationships that emerge, the narrative takes a somewhat darker tone in examining the complexities of life, as witnessed in 'The Dark Age', 'Innocence', 'Passion', and 'Becoming Pts 1 & 2'.
Season 3, likewise, advances the character arcs. We get a better sense of the increasing power struggles between many of the characters (i.e. Buffy/Faith, Buffy/the Council, Angel/Himself). This season is filled with both humor and consequence.
I understand Season 4 is widely regarded as the worst season. Sure, there is a lack of interest many of us have with the Initiative and Riley. However, this season brought me several of my favorite episodes: 'Hush', 'Something Blue', 'Wild At Heart', and 'The Yoko Factor'. Moreover, I think this season, although not my favorite, has more good episodes than the others.
Season 5 was quite a departure from the previous years. Buffy has a growing interest in her heritage, Dawn is at once human and mystical energy, Spike becomes, more or less, a supporter of the Scoobies. But aside from the last five or six eps of the season, I found it missing something.
Season 6, I have to say, is probably my favorite. I could go on for a long while about this, but I'll just say that S6, although dark and depressive, was ultimately what the show is about: humanity. If you really want some detailed explanations about why I love S6, then check out some of my posts in the S6 section.
Alright then, thanks for letting me ramble for a long time (if you're still reading).