6DOIT doesn't count as one song here, since I think any sane person would pick a 45 minute song lol.
Anyway, for me, it's either A Change of Seasons or Octavarium (not much of an original choice, but oh well). I would need a song that's very emotional, thought provoking, and filled with variation, which these two songs have. Personally, I find that I have a real connection with these two songs on an emotional level, and if I had to pick one song, I'd need one I could relate to.