I think that it's a coin flip between "Passion" and "I only have eyes for you" (which also happen to be two of my all time favorite episodes). Angel (or I guess Angelus) narrating "Passion" was such a perfect stroke for that episode. It added to the feel tremendously and contributed to pretty much the entire complexion of the series changing. A little bit of innocence was lost, truly lost, in that one. Jenny finding out how to restore Angel's soul, only to die in that amazing chase scene through the school was incredible. If we're talking great artistry, I absolutely love the image of Angelus, in all his vampiric glory, warming his hands over the fire. That was a great shot. Giles discovering Jenny's body, and the lead in to it, was such a powerful scene, and Angelus watching Buffy and her friends just crumble as they receive the news was so haunting. It's really the first time that we sense that the Angel we knew is gone, or at least it was for me.
"I only have eyes for you" though, I think equals "Passion" in terms of great artistic success. Buffy and Angel mirroring the two dead lovers from the 50's, only with the roles reversed, was so beautiful and sad at the same time. The fire that Buffy has that entire episode until the climax, and the way that it ends, with her and Angelus passionately kissing, even when they're both freed of the ghosts, was just so, so, so good. And the final shot, of Buffy asking saying "Angel", where she's just so vulnerable and wants it so bad, is so heartbreaking. I love that episode.