I voted for "Standing" as I think that was such a beautiful song, establishing Giles' decision to leave as well as his desire to be a surrogate father for Buffy. I also haven't seen it get many votes which doesn't surprise me as it's probably one of the shortest.
Secondly, it would have to be "Under your Spell" as I believed Tara and Willow's relationship to be extremely sweet and textured. It also foreshadows Willow's deceit because Tara is "Under her Spell" due to the Lethe's Bramble Amnesia spell.
Then "Rest in Peace" would come a close third followed by "Going Through the Motions", "I've Got a Theory", "I'll Never Tell", "Walk Through the Fire", "Where do we go from Here" and then the rest in no particular order.