What is your favorite guitar part in a rock song? For me it’s Pink Floyd’s “Is There Anybody Out There” because I could probably just listen to that song looped for hours and not get sick of it.
I was trying to think of any non-Floyd to list, but I can’t do it. And there is so much great stuff that Gilmour did. But I’m going to go with the opening riff from Sorrow.
If we could stretch the question to jazz-fusion, then the first guitar note from Michael Mantler’s Something There.
Either way we’ve got Nick Mason on drums.
Way, way, way too many to try to break down. 
The Southern fried boogie three guitar riff at the end of the immortal Free Bird.
Far too many for there to be a “favorite” per se but the first thing coming to mind is The Outlaws’ Green Grass and High Tides (Forever).
Hmmm… off the top of my head:
In Hendrix’s version of “All Along the Watchtower” when the wahwah solo comes in (with some delay I think), I just find that moment and that solo perfect.
Beatles, Abby Road, the 3-way trading solos in “Carry that Weight”.
Around the time I first started playing (some 23 years ago), I loved the intro riff to GnR’s “Sweet Chile 'O Mine”, so much I actually did make a cassette tape looping just the intro so I could keep listen to it. It was one of the first things I learned to play.
Lots of Steve Howe moments on Yes’ “Tales from Topographical Oceans” though too many to name (and too difficult to describe the moments anyway since each track is 20 mins.)
Last 2 tracks of Ozzy’s “Diary of a Madman” album with Randy at his best. (except hate the guitar solo section in the song “Diary…”. A very weak section for a very strong song).
Too many from Zeppelin to name. (and the truth is I haven’t listened to this music in so many years it’s hard to remember names).
I’ve always been partial to the guitar breaks in Alice Cooper’s “Caught In A Dream”. Nothing fancy, but enough for me to get my air-rhythm guitar every time I hear it.
The middle solo in Pink Floyd’s “Mother”, 30 seconds of transcendental genius.
Allman Brothers Band at the Fillmore East, Stormy Monday – all guitar work is superb.
By the way OpalCat - are you referring to the song **Comfortably Numb **off the Wall? I assume so - it starts off with “Hello, hello is there anybody in there…” and the guitar lead work on it is consistently held up as some of the best ever (and, as a guitar geek, I love the fact that Gilmour did NOT use his signature Stratocaster for that lead; it was a 50’s Les Paul with old soapbar P-90 pickups…).
Okay - here’s a couple:
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AC/DC, Problem Child: just launches with bone-headedly simple chords that sound great and set up a perfect rock groove.
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Rolling Stones, Can’t You Hear me Knockin’- pretty much the greatest riffy intro in rock and roll. He’s all over the place with the beat and sloppy as hell, but that is part of what works. Doesn’t get any better.
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UFO, Rock Bottom- Michael Schenker just kicks ASS on this intro and riff (and the full lead in the middle of this live version is great, too). Think Randy Rhoads and Yngwie Malmsteen listened to a little of this??
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Chuck Berry, Johnny B. Goode- the Alpha and Omega.
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The Hollies, Long Cool Woman- about 1:20 in. Just a great, fun riff.
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**James Brown, the Big Payback **- riff starts about 40 seconds in. You have no idea how simple this two-string riff is, but man, it is the back bone of this song…
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U2, I Will Follow You have no idea how simple this two-string riff is, but man, it is the back bone of this song…and I loved being able to cut and paste that line…
The Snowy White/Rick DiFonzo guitar duel on Comfortably Numb during the Berlin Live Version of The wall. I get chills every time I hear it.
David Gilmour’s solos in “Shine On You Crazy Diamond”
… plus Slash’s solo on GNR’s November Rain
My first inclination was to answer like WordMan, but I’ll mention a few:
Keith Richards: intro to Can’t You Hear Me Knockin’ and Honky Tonk Women
Ron Wood: intro to I’m Losing You
George Harrison: Something and Octopus’s Garden
Eric Clapton: Crossroads and Badge
Jeff Beck: I can’t even reduce it to a handful
Eric Johnson: Cliffs of Dover
Mark Knopfler: Sultans of Swing, Telegraph Road, Boom Like That, see Jeff Beck
Freddie Jones Band: live version of Already In A Daydream
Angus Young: Back In Black
Jimi Hendrix: Watchtower, Little Wing, Purple Haze, see Jeff Beck
Los Lonely Boys(Henry Garza) intro to Heaven
I need to stop now.
The 4-note riff towards the beginning is near the top of my list.
The song “Is There Anybody Out There”:
Totally different song, although both are great.
Duane’s solo in that song, and his solo in Elizabeth Reed (same album), are my two favorite guitar performances. Ever. By anyone.
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Stairway to Heaven: it was the first one… then came many… then I’m back to feeling and smelling the same air again. What’s there to talk about so much, I wouldn’t know.
One that sticks out in my mind is the stop-time break that precedes Martin Barre’s main guitar solo in Jethro Tull’s “Aqualung.”
Robin Trower on “The Milk of Human Kindness” off Procol Harum’s A Salty Dog.
Jeff Beck’s solo in the middle of “Shapes of Things” by the Yardbirds (like Crotalus said, it’s hard to reduce to a handful, but this was the first electric guitar solo that made me shiver!)
Keith Richard’s solo on “Sympathy for the Devil.”
Mick Taylor on “Midnight Rambler” from Get Yer Ya Ya’s Out.
Peter Green on Fleetwood Mac’s “The Green Manalishi.”
Duane Allman after the crescendo of “Whippin’ Post” off The Allman Brothers Band.
Lowell George’s slide on “Oh Atlanta” from Waiting for Columbus.
Eric Clapton on “Crossroads.”
John Cipollina all the way through “Who Do You Love” off Happy Trails by Quicksilker Messenger Service.
I’ll stop now…