Vgonis, i don't want to attack Quo. It's a good Rock band, they have been playing together for years. I've listened to several albums. They did a good job. Still i don't think that any of their songs has a better rock riff than money for nothing.
Anyway, i don't like polls that much, and there were no categories defined what a "good" guitar riff is. So it's not clear wether a good riff is (1.) a riff that is easy to play so that everyone can replay it (2.) a riff that shows musical creativity (3.) a riff that requires high technical skills (4.) a riff that makes people move (5.) a riff that is getting famous and copied by many follow bands (6.) a riff that is immediatly recognisable (7.) a riff that brings nice memories (8.) a hard, soft, romantic and so on riff.
in my opinion a good riff should be created by the artist, should be iconic (recognisable) for the song and should have a wow effect. This Wow-effect i had when i first listened to the Money for Nothing - Riff, i don't have it all when i listen to Down Down. (Whole lotta love, Back in black, Smoke on the water, Layla are good and iconic riffs, very recognisable), Sweet child o#mine is a good riff but imho a little bit overrated. So i don't have a problem that these songs are under the top10. - But Down Down and How soon is now? - Am i the only one that thinks these two are overrated (better than Money for Nothing? in comparison with all the other named riffs) - I can't believe it...