For me it's Gimme Shelter by the Rolling Stones

I also like She sells Sanctuary by the Cult (long version) and Do anything you wanna do by Ediie and the hot rods.

As for riffs, to many to choose from.

90% of Northern Soul tracks including:
Bari Track - Doni Burdick

Can you hear me knocking by the Rolling Stones
Stay With Me by the Faces
5:15 by the Who
Carry On by Crosby Stills and Nash
Fresh Garbage by the Spirit
Roadhouse BLues by the Doors
For those about to Rock ACDC
Reelin in the Years by Steely Dan
Pump it Up Elvis Costello
there are way too many to mention

Back in black AC/DC
its a long way to the top if you wanna rock'n'roll AC/DC
Iron man Black Sabbath
smoke on the water Deep Purple

Forgot
The Trooper Iron Maiden

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.

'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.

Off the top of my head

Breadfan by Budgie
Du Hast by Rammstein
Sundown by Slam Cartel

One I never tire of.

Running Free. Maiden
Crazy Train. Ozzy
Who made Who . AC/DC.
Iron Man. Sabbath.

Mr Crowley, Crazy Train, I don't Know and my favourite Diary of a madman - Ozzy Osbourne with Randy Rhodes on Guitar

The title track from Diary of a Maddman by Ozzy is a superb riff. The orcestral work on that track is still un-match in hard rock to this day IMO and over 30 years old ! Randy Rhodes was on fire on that album.