This should probably be in the stolen topics thread because i heard it on ron and fez, but i love getting into discussions about the definitive songs by an artist or band.
For instance, it's the song that culminates the essence of the band. It's what you tell someone to listen to if they've never heard the band before to get an idea of what they're in for. This can also be done with albums, but they are usually too obvious.
Here are some bands to start you off:
Grateful Dead - Dark Star
Pink Floyd - Breathe (no, not another brick in the wall)
Nirvana - Teen Spirit (sorry but it is definitive)
Pearl Jam - alive
what have you got?