Definitely "Becoming". I mean Willow and her "determined look", and that outstanding performance of Alyson Hannigan when she feels the spell coming through her or whatever. I mean, Xander being a complete bast*rd - once again in these three first seasons... Buffy fighting and finally killing her love cos she has to. Kendra dying, by Drusilla finally showing us the extent of her power... Giles being tortured... And Snyder being such an asshole he cannot not remind me of the French president (sorry...)... And Buffy's life totally falling apart, with Joyce learning it and... Well, you just feel something's changing. That's the greatness of every good season finale. I just love that episode.
But that episode is so good it just cannot count. So, right after this episode, I think I'll go with "Passion". Angel narrating, Jenny dying in this violent, brutal way ; and even more brutal and violent is the scene when Giles discovers it. Willow and Buffy falling down when they learn it... I loooved Jenny for God's sakes!