Layla - Derek and The Dominoes
American Woman - Guess Who.
Spirit of radio…RUSH. 
Scorpions, The Zoo. Got it as my ring tone.
Tom Petty And The Heartbreakers - Runnin’ Down A Dream.
Tricky one, this. So, so many to pick from.
I`ll start with Bridge Of Sighs - Robin Trower.
…or maybe La Grange - ZZ Top!
This is a good one. “Smoke on the Water” (Deep Purple )
Hells bells, ac/dc
Thin Lizzy - Whiskey in the jar
Sweet Home Alabama … Lynyrd Skynyrd …
Big Chris:
Hells bells, ac/dc
Great song by a fantastic band but I wouldn’t say it was the best opening guitar riff ever. It starts with bells ringing for the first 20 seconds.
Now if you’d said Back in Black…
AC/DC - Let There be Rock
Jon Spencer Blues Explosion - Blues X Man
McLusky - whoyouknow (here set to clips from “Bottom”, which works surprisingly well)
Speedball Baby - The Jack Martin Story
Blur - On Your Own
Cool Jerks - Man and a Woman
Chuck E Weiss - Devil with the Blue Suede Shoes
and Half Man Half Biscuit - Thy Damnation Slumbereth Not (not strictly a guitar opener, but such a great riff when the bass kicks in that I couldn’t resist it.)
Enjoy!
Money for nothing.
Dire Straits.Mark Knopfler is a legend.
tamarman:
Sweet Home Alabama … Lynyrd Skynyrd …
■■■■ right!
Tears for Fears;
Change.
Pale Shelter.
Everybody wants to rule the World.
Big Country;
East of Eden.
Steeltown.
In a Big Country.
Sweet child of mine. G&R
Ken.
Money for nothing is a good call Toby, probably second to my OP in my opinion at least.
Some - in fact, most - of the earlier Dire Straits is really good too. I don’t need to hear Knopfler playing riffs … I could listen to him playing one note all day long. The tone he got from his Stratocaster in the early days was beautiful.