Easy:

"Change" by Blind Melon. I think I've listened to that song - and I do not exaggerate - at least 300 times (I've listened to their eponymous album at least 200 times). I will never, EVER, get tired of it, and it moves me every time I listen to it.

There are few other songs I can think of that I would never ever tire of. A more recent one for me (recent being the past few years), is "Monsters" by Band of Horses. That song is gorgeous! It has an incredible build. I listen to it every time I'm feeling down, and it somehow picks me up again.