Aaaah, that's a very difficult question. I have pretty inclusive tastes, so it often depends on my mood. Going by the most played on my iPod doesn't really help much.
However, I think the band that had the most presence and meaningful impact in my adolescence was Modest Mouse, so I'd say:
http://www.youtube.com/watch?v=U6XhVj5GF0I
As for the reason why, I suppose it's just that a lot of their songs encapsulated a whole bunch a feelings that happened alongside the time I found them.