Selling England by the Pound is also one of the best. I can't help if my choices are generic, but I'm not prog expert and I have just start my journey trough the sea of prog rock. Also the classic are usually classic because they deserve it. (Not always, of course, but often).
Maybe Thick as Brick is better, I'm not sure. At least Dancing With The Moonlight Knight is the best prog song ever recorded (or the best one I've heard, tho' it is not telling much.
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