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.

Second best, Gimme Shelter is better

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

Good Call but I think I'll stick with my favourite. Each to his own I suppose.

How about " When the Levee Breaks" by Led Zeppelin? Rock version of Delta Blues. Pretty special.