Can't really decide... definitely Still a Weirdo by KT Tunstall though.
YouTube - KT Tunstall - '(Still A) Weirdo' Lyrics Video (HD)
And probably Heard the World by OAR...
YouTube - Harry Potter- Heard the World (O.A.R)
(sorry for the fan video, but youtube only has Live performances or covers. /: )
And maybe even Hero by Regina Spek tor
YouTube - 500 Days Of Summer (2009) Soundtrack 11- Hero