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Shine On You Crazy Diamond, prog rock? Hardly. It's derivative blues, there's no harmonic or thematic progression. Even the idea of isolating the prog-rock guitar for special praise is self-contradictory as prog is much more about the ensemble.I'd start with Steve Howe as the definitive prog-rock guitarist, but even then it's difficult to isolate a particular track that epitomises his art - say Yours is no Disgrace, but it's just an example.And it would only be right to mention Robert Fripp too, but if I picked my favourite King Crimson track, Indoor Games, his guitar, though defining and essential, is hardly ostentatious.Same with just about any prog act I can think of (Genesis, Gentle Giant, Jethro Tull, ect) - there are guitars there, but it's just not easy to define prog in terms of its lead guitar - that's a defining characteristic of rock and blues, not prog.