The Intro to Enter Sandman i always liked.. when i first heard that song i thought hmmm this is going to be interesting.
The way they construct the main guitar riff of the song note by note in the intro itself. If your a guitar player and you know how to play the song you'll know what i mean. The whole song is basically written around one scale.
The intro to Smells like teen Spirit is one of the most dank, dirgy, depressive, things i've ever heard. Very foreshadowing of whats to come.
As for intros i've got some pretty cool ones myself.....
Check out my song "Pig Killer'
http://www.myspace.com/groovegrinder