It can be a solo, or part of the song, but it must kick things off.
“And Your Bird Can Sing” - The Beatles
“I’ve Just seen a Face” - Bealtes
“Heart of the Sunrise” - Yes (here - it’s rhythm guitar, blending in with everything else, but still rips)
Nothing made this 5-year-old back in '69 bounce off the walls more than this opening:
“Up Around the Bend” - CCR
M.E.W. -
For the classic, meat-and-potatoes HEAVY opening, it’s gotta be:
“Symptom of the Universe” - Black Sabbath
When Doves Cry.
Can’t You Hear Me Knocking.
Watching The Detectives.
And though you’ll never see me at a Billy Joel concert anytime soon, I love the opening riff of Big Shot. I feel like that’s one of his “least-Joel-like” songs…sounds like it could have been done by a different group, but for his voice.
I almost feel like Roxy Music/Bryan Ferry could have done a really badass version of that song. Something along the lines of Grey Lagoons.
All The Young Dudes by Mott The Hoople
Way too many to list, but let me start with…
HELLS BELLS.
Crazy On You by Heart. Could post that link in the “female guitar players” thread, too.
Brown Sugar
Also: Adrian Belew’s guitar synthesizer “horns” opening Paul Simon’s You Can Call Me Al.
Day Tripper by the Beatles
Honkey Tonk Women.
The Confessor
Wanted Dead or Alive…
I’ll be back:D
Okay, here’s my list of favorites:
Brown Sugar
Adrian Belew’s guitar synthesizer “horns” throughout Paul Simon’s You Can Call Me Al
Statesboro Blues (Duane Allman Brothers Band live)
Out On A Limb (Johnny Winter)
Killing Floor (Jimi Hendrix at Monterey)
Roll Over Beethoven (Beatles)
Touch Me (Doors; the opening is layered with organ, harpsichord, drums, and bass, but it’s primarily a guitar riff)
Walk Away (James Gang)
Boney Moronie (Johnny Winter live)
Elephant Talk (King Crimson; Tony Levine playing a Chapman Stick which I’m counting as a guitar)
Be With Me (The Butts Band)
Magic Fingers (Frank Zappa)
The groovy beginning of “Drive My Car” (Beatles) makes me wanna go out and by a cheap Corvair convertible.
Pink Floyd - Sorrow
IMO, the answer to most questions like this is always: Gimme Shelter.
Whole Lotta Love
Some that come almost immediately to mind:
Carlos Santana & John McLaughlin - “A Love Supreme” (written by John Coltrane)
Cream - “Steppin’ Out”
Ten Years After - "I'm Going Home" Johnny Winter - "Be Careful with the Fool" Led Zeppelin - "Jennings Farm Blues"
Mississippi Queen:cool:
Kickstart my heart
Red house
Cliffs of dover
Bohemian Rhapsody, because its opening guitar riff is nothing at all, the song just starts from nothing.
Also Sprach Zarathustra (the famous orchestra bit from 2001 A Space Odyssey), because huge. (Even though it’s so overused, I don’t care)
The C minor organ passacaglia by Bach, because that ominous 15-second bass riff goes non-stop for 8 minutes to build something incredible. (Plus it helps when the bass you’re playing is 32 feet tall) 
The intro to “Limelight” by Alex Lifeson (with Neil Peart jumping in near the end) - clean and catchy, and not terribly technically-challenging, especially for a guitarist like Lifeson, but it always grabbed me.
Boston’s “Foreplay” (intro to “Long Time”).
Triumph’s “Rock Out, Roll On” - love the way the two guitar lines commingle. Their instrumentals too (“Petite Etude,” “Midsummer’s Daydream,” “A Minor Prelude”); Rik Emmett is clearly classically-trained. Very much an underrated band.
The Eagles’ “Life in the Fast Lane” - good old hard-driving rock ‘n’ roll.
Clapton’s “Same Old Blues” - rhythm and flow interspersed with a cool lick. And, of course, “Layla.”
Those are what I can come up with off the top of my head.
Heh - I was gonna post:
Alex’s effects-phased trills? arpeggios? triplets? at the beginning of Rush’s “Spirit of the Radio”.
One could argue that it just sounds like an exercise.
Well, ok - a cool one, then!
NOT “Pictures of Matchstick Men”
Nothing technical - just pure, raw, darned catchiness: “Crusier’s Creek” - The Fall. (possibly the greatest drunk video ever made - enjoy) (R.I.P. annoying Mr. Smith)