Vicarious by Tool.

IT PWNS U!!!!!!

Seriously though, I cannot for the life of me put my finger on one ultimate song. There are just so many that I love.

Some that could qualify are:

Red Hot Chili Peppers - Under The Bridge
Stairway to Heaven - Led Zeppelin
Jeremy - Pearl Jam
Give to Fly - Pearl Jam
Master of Puppets - Metallica

ehn........I dunno.....there's more, but I'm not about to list every song that I enjoy to that extent