Only just spotted this thread... Personally I'd find this interesting, but agree it doesn't really fit in with what RG do...
As far as what I like, demos such as Enigma's "Phenomena", Alcatraz's "Odyssey", Kefrens' "Desert Dreams" and Mad Elks' "Technological Death" are ones which still stick out in my mind, and I still remember the tunes of.
Melon Dezign had some interesting quirky little demos as well.
A lot of Amiga users always mention State of the art, 9 fingers and Jesus On Es as some of the top demos, but IMHO they were really just slightly elaborate animations rather than anything pushing the boat out technologically (although the music from Jesus On Es is utterly, utterly awesome).