Julian Cope presents Head Heritage

They've probably all been mentioned here, but I'd go for: Egg, Henry Cow, Van der Graaf Generator (particularly "Godbluff"), Pavlov's Dog, Massacre (prog or avant-garde? hmm), Magma, Comus's first album, Caravan ("In the Land of Grey and Pink"), Agitation Free (more Krautrock, maybe), Soft Machine (again, is this avant-garde?), Kevin Ayers, Robert Wyatt's "Rock Bottom", any King Crimson, especially the debut, and I have to admit to having very soft spots for "Dark Side of the Moon", "Close to the Edge", "Tarkus" and "The Lamb Lies Down on Broadway" by those devils Pink Floyd, Yes, ELP and Genesis respctively. I know, shame on me! Plus, there's a whole wealth of proto-prog that's worth checking out, such as The Moody Blues, Writing on the Wall, "Hot Rats"-era Zappa or avant-garde psych bands like The United States of America and The Residents.