yes, Lamb is the best Prog album in my opinion. Gabriel took total control of the story and lyrics, which was needed. And wrote some of their best material ever. It had a concept, better than Tommy or Yes Tales From Topographic Oceans. And Gabriel’s voice and articulation of his rhymes were brilliant. Also creative contributions from Eno.
It’s a shame that in all of the Genesis docs you see the rest of the band members are always downplaying the album mainly because of Gabriel’s conceptual control and his costumes for the live performance. They always talk about the technical failures of the tour and not the album itself. Tony Banks is the main naysayer and the one band member to have failed at his solo efforts. The album is brilliant despite their self evaluation, especially the Tony Banks instrumental sections.
The competition in my mind is something by Yes, my pick would be Tales From Topographic Oceans, for the depth and instrumental arrangements. Howe and Squire went crazy on this one, Waksman was a little more subdued. And Alan Watts turned out to be a more palatable drummer than Bruford, who might be technically better. A lot of people pick Close to the Edge as a best Yes album, but I find between the nice parts that there’s a lot of noise and unpleasantness.
King Crimson Red is another pick of mine. The most focused Crimson album with the absolute best lineup of the band. It’s darker than other Prog bands, with no discernible English folk or American roots influence. Crimson probably had more proto metal sounds than Sabbath.
Van Der Graaf Generator’s Pawn Hearts is another pick of mine. They definitely sound like no one else and scoffed at being lumped in with Prog rock just as Robert Fripp did.
Jethro Tull made two overtly progressive albums; Thick as a Brick and A Passion Play. I would pick the latter as one of my favorite Prog albums.
Gentle Giant has to be in there somewhere as they are the definitive Prog band in every way. It’s hard to pick one album from them but Octopus, The Power and the Glory, and In a Glass House are all up there.
I also agree that Radiohead’s OK Computer is a great Prog album in every way.