I thought you had watched it before, but I was just checking! Probably season 6 -- you get the musical episode, Normal Again, the three geeks of the apocalypse, the whole thing with Tara toward the end. Although, that might be outweighed by Doublemeat Palace and Willow getting hooked on magic.

Although, seasons 3 and 4 were pretty darn good. Three had Faith, Band Candy, The Wish, The Zeppo, and the Mayor, who might be my favorite villain. And four had Something Blue and Hush, although it had the initiative and Beer Bad. So maybe not four. Hm.

So I guess it's between 6 and 3 :-) Maybe I should just rewatch them all again to make sure! ;-)