I will remember you is my favorite, no doubt. That being the first time they mixed after the shows split, how emotional it was, I can't think of a sadder yet more perfect way to describe the whole Buffy/Angel relationship than that episode. The entire middle leg of that episode, when you know that it's not going to last but it's just perfect while it does, is some of my all time favorite Buffy and Angel (huh, that sounded a little sappy).
Plus, the whole notion of Angel being the only person to remember the day just suits him so well.
Love that episode.