Good question, but I've never thought of it that way. As others mentioned, classic prog usually incorporates more classical elements (or jazz as with King Crimson), and The Who's blues/R&B roots give everything they did a different slant. But Townshend's use of synthesizers beginning with Who's Next and the grandeur of Quadrophenia certainly edge toward prog.
I've never really thought of Pink Floyd as prog either, but they probably fit. Meddle definitely does, and is one of my faves.
Some of my others would be:
Yes: Fraglile and The Yes Album
King Crimson: everything mentioned already, especially Lark's Tounges and Red, plus Discipline and Thrak (an underrated masterpiece IMO)
Mars Volta: De-Loused in the Comatorium
Tool: Lateralus
Radiohead: Kid A, their most "prog" that's listenable IMO
Rush: Moving Pictures
+1 on De-loused in the Comatorium. One of the best albums of the last decade.
