I won't even get started on my favourite Dream Theater riffs or this list will go forever and it's probably going to end up long enough as it is. These are just off the top of my head:
A.C.T "Truth Is Pain" (outro riff = pure bliss)
Iron Maiden "Be Quick Or Be Dead", "Afraid to Shoot Strangers"
Vanden Plas "Push"
Angra "Live and Learn", "Speed"
Shaman "Time Will Come", "Iron Soul"
Helloween "First Time", "Departed" (simple but very atmospheric)
Children of Bodom "Better Off Dead"
Alice Cooper "Bed of Nails"
Iced Earth "Phantom Opera Ghost"
Megadeth "Tornado of Souls"
Steve Vai "Down Deep Into the Pain", "Building the Church"
Joe Satriani "Bamboo"
Metallica "Holier Than Thou"
Rage "Paint the Devil on the Wall", "Soundchaser"
Saber Tiger "It's Your World", "Sleep With Pain"
Twilightning "Gone to the Wall", "Plaguehouse Puppet Show", "Fever Pitch"
Pantera "13 Steps to Nowhere"
Rusty Cooley "Dark Matter", "War of the Angels"
John Petrucci "Glasgow Kiss", "Jaws of Life"
Haji's Kitchen "Free"
Thunderstone "Liquid of the King"
Psycroptic "Cruelty Incarnate", "Alpha Breed"
AC/DC "Back in Black"
Guns N' Roses "Welcome to the Jungle", "Locomotive", "You Could Be Mine"
Deep Purple "Anya", "Sometimes I Feel Like Screaming"
Masters of Reality "Kill the King"
Slash's Snakepit "Be the Ball"
Van Halen "Ain't Talkin' 'Bout Love", "Pleasure Dome"
Blue Oyster Cult "Take Me Away"
Saber Tiger "Red Shadow"
Dire Straits "Sultans of Swing", "Money for Nothing"
John Farnham "All Our Sons and Daughters", "Diamonds"
ZZ Top "La Grange"
Buckethead "Park Theme", "Skull Scrape"
Racer X "Fire of Rock", "Superheroes"
Aghora "Atmas Heave"
Friar Rush "Abduction", "Eve"
Gordian Knot "Muttersprache" (simple yet freaky intro)
Lemur Voice "Akasha Chronicles"
Liquid Tension Experiment "Paradigm Shift", "Acid Rain", "Another Dimension"
Planet X "Poland"
Symphony X "In The Dragon's Den", "Evolution", "Absence Of Light", "Inferno", "Eve of Seduction"
I could probably think of more favourite riffs but I'm still in 24/7 Dream Theater and Iron Maiden mode