Hum. I don't know if there is a song that defines my personality or anything, but I consider the following to be "my songs" so to speak...

Five For Fighting - Superman
the goo goo dolls - Name

And to a lesser degree...
MIC - Superhuman
Crowded House - Weather With You
Fall Out Boy - Nobody Puts Baby In The Corner
the goo goo dolls - All Eyes On Me
Go West - The King of Wishful Thinking

But definitely Name and Superman would be my top ones, by a long way. Superman has been my 'theme song' for a longer time, but the sound of Name rather than the lyrics is what reminds me of myself so much. I don't really know why.