(Opinions expressed are of course just DWs personal own and reflect nothing/nobody and etc.)
"Omensetters Luck" by William H. Gass (1966)
Gass' first novel, and his least avant-gardeish, and his best. Basically a religious book. Very sad. Contains the immortal line "The body of Our Saviour shat but Our Saviour shat not." Bleak but gorgeous, like light through ice.
"Steps" by Jerzy Kosinski (1968)
This won some big prize or other when it first came out, but today nobody seems to remember it. "Steps" gets called a novel but it is really a collection of unbelievably creepy little allegorical tableaux done in a terse elegant voice that's like nothing else anywhere ever. Only Kafka's fragments get anywhere close to where Kosinski goes in this book, which is better than everything else he ever did combined.
"Angels" by Denis Johnson (1983)
This was Johnson's first fiction after the horripilative lyric poetry of "Incognito Lounge." Even cult fans of "Jesus' Son" often haven't heard of "Angels." It's sort of "Jesus' Son's" counterpoint, a novel-length odyssey of mopes and scrotes and their brutal redemptions. A totally American book, it's also got great prose, truly great, some of the '80s' best; e.g. lines like "All around them men drank alone, staring out of their faces."
"Blood Meridian: Or the Evening Redness in the West" by Cormac McCarthy (1985)
Dont even ask.
"Wittgenstein's Mistress" by David Markson (1988)
"W's M" is a dramatic rendering of what it would be like to live in the sort of universe described by logical atomism. A monologue, formally very odd, mostly one-sentence 6s. Tied with "Omensetter's Luck" for the all-time best U.S. book about human loneliness. These wouldnt constitute ringing endorsements if they didnt happen all to be simultaneously true -- i.e., that a novel this abstract and erudite and avant-garde that could also be so moving makes "Wittgenstein's Mistress" pretty much the high point of experimental fiction in this country.