Voted Pull Me Under. I feel like it indicates what the sound of the rest of the album will be like, while at the same time it gets you moving and grooving. 6:00, New Millenium (gasp!), and OTBOA also do this very well.
I notice The Glass Prison has a huge lead in the poll. While I totally agree it is an awesome song, and is one of their top notch album openers, I just don't feel it sets up the REST of the album quite as well as the others I mentioned. It's almost as though the song is such an experience in an of itself, anything afterwards is something seperate.