good choice, i will add to that Powerslave, Women in uniform & Moonchild

Radiohead - Just (Also has a very impressive outro too!).

And despite some of you mentioning The Who, nobody mentions:

Baba O'Riley

Best intro of all time

Magazine -shot by both sides .

Mt Abraxas (by Uncle Acid & the Deadbeats) - about 4.5 minutes in a massive doom riff kicks in for about another 2 minutes solid. I think its the riff of the decade. It basically out-sabbaths sabbath if you know what I mean.

So come on by The Atomic Bitchwax has a kick ass riff, also Evil eye by Fu Manchu

Intro - Sweet Child O' Mine played loud, and I guess that covers a pretty good riff too.

Run till it's Dark - Public Enemy

Behind The Lines (especially live) - Genesis (written by the man in the Avatar)

another vote for Gimme Shelter

Let There Be Love - Nat King Cole

Tie Your Mother Down - Queen

Layla - D & T D

and for a menacing start, Intruder - Peter Gabriel

'Welcome To The Jungle' - Guns N' Roses

mute picking mixed with an echo device to simulate sixteenth notes - pure genius from Slash.

Rebel Rebel - Bowie.

Monkey Knife Fight by Black Nasa.
Last of the V8 Interceptors by The Atomic Bitchwax.
Mud Crusher by The Mushroom River Band.
Ride a Black Horse by The Kings of Frog Island.
Witches Chasing Cabs by Puny Human.
Cocaine Rodeo by Mondo Generator.
Chemical Lady by Sasquatch.
No Good Woman by Radio Moscow.
Mississipi King by Five Horse Johnson.
Jim Beam & Good Green by The Glasspack.

The Smiths - What Difference Does it Make?

Game over.

'Barbarism Begins At Home' a very close second.