Day 3: Favourite Song Used In An Episode
I'm going to interpret this question not to mean a song especially written for an episode because that would either be "Under Your Spell" or "Walk Through the Fire" ("I think this line's mostly filler" cracks me up every time) but for anything played or heard on the show.
There is an awful lot of songs to choose from but the first artist that came into my head was Aimee Mann because I'm a bit of a fan. I was beyond thrilled when I saw Aimee Mann on Buffy, she sang two songs in the Bronze and got a line! Nobody else got a line. Love her.
Of course, BtVS has one of the best ever theme tunes.