Holy crap, I had no idea there were other Buffy fans here. I knew slayers_creek_oth was one but that's all. (Brokeback and Buffy are my two favorite things, so this is really great.)
I got to say, Passion and Once More, With Feeling are both amazing episodes (in completely different ways). Passion is incredible because of it's excellent and grim direction and the actors were phenomenal in that episode. And I love OMWF for Joss Whedon's excellent writing and directing and that the actors did their own singing.
Although, my favorite episode is the The Gift. The entirety of the episode, for me, was really gripping and heart-tugging and the final scene was incredible.
I must say, though, I have about 50 other favorite episodes.