My immediate top three has always been Restless, Once More With Feeling, and Fool For Love (in that order), those are all stand-out episodes IMHO that never get old. And then hmmm I'd probably say Prophecy Girl as I have a real soft soft for that episode and how it brings the first season to a close, Innocence for being such a game-changer for the show, The Wish for the fun of seeing the characters in such a dark AU, and maybe Who Are You for Sarah channelling Faith?