In The Wake Of Poseidon, Lark's Tongue In Aspic, and Starless and Bible Black.
I was more familiar with In the court..., Red, Discipline, Beat, Three of a perfect Pair and I remember seeing a live BBC video a long time ago of Lark's Tongue I or II. I am less familiar with the newer stuff. I got a long way to go with them. But it's hard sharing time with them and Brian Eno and his work with Fripp, and Camel, and other old school prog and new school prog like ZH and Ayreon, and my Stoner Rock/Doom bands, and my growing fascination with different classical composers.