Maybe not the 'greatest', but pretty good.
Sweet - Love is Like Oxygen
(Yeah, the same guys that did "Little Willy" and "Ballroom Blitz".)
Especially starting at about 3:09. Before that it sounds a bit like 10CC.
But one of the greatest might be Eberhard Weber "No Motion Picture" from the "Colours of Chloe" album. This 19 minute masterpiece moves between several genres seamlessly. But it is definitely proggy.
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