Season One. I only have one episode listed for the entire season but the season itself was a strong one with lots of clever dialogue and getting to know you situations for our main characters. As the years went on the series became even stronger but in the fifth year the series crashed and burned despite having a tremendous villain in the form of Glory. Of course "crashing and burning" when you're talking about Buffy still leaves you with a show that's much better than almost everything else on tv.
Season Five. While Glory was a wonderful villain, this season marked a down turn in the overall quality of the series. Still they did manage to turn out stunning episodes like this one in which Buffy comes home to find her mother dead of natural causes after having resolved her previous medical issues. This was as powerful as anything ever presented on television.
Season Six. A perfect example of what the show could do when it was working right. Seasons 5, 6, and 7 were all angsty and overwrought and almost completely devoid of the humor that drew us to the show in the first place. But in this delightfully creative musical episode Joss Weedon remembered how he used to be able to put humor and angst together instead of making them mutually exclusive.
Season Six. Willow casts a spell hoping to make everyone forget the bad things that have happened. Instead they forget who they (and everyone else) are, and they have to piece things together from what they see around them.
Season Seven. Willow hears from Tara, or does she? Dawn's seeing her mom, or is she? And Buffy is about to stake a newly risen vamp when he recognizes her and they sit down for a heart to heart talk before they fight to the death.