Quite honestly I'm stunned, no, I'm MORTIFIED, that "The Body" isn't Number 1 on everyone's list. It is without a doubt a Top 5 episode (along with OMWF and Hush) and arguably the best episode of BtVS. It could be argued that it's the best episode of TV ever. People, it's that good! Do I want to watch it today? No... I'd rather watch Buffy v. Dracula because I'm in that kind of mood.
Number 2 should be "The Gift," however I don't feel as strongly about that as I do "The Body." The Body is perfect and deserves to be Number One.