Hooboy...
Okay, first of all, one of my favorite authors: Kurt Vonnegut, Jr. I agree he's great. But science fiction? No way. Doesn't belong here. Labeling Vonnegut's work as SF is like labeling Edmund Hamilton's work as literature. There are similarities, but no way it qualifies.
Authors I would agree belong on the list of excellent writers who just happen to be in the SF field: LeGuin, Dick, J. G. Bllard, and I'm definitely tempted to add Sturgeon. Not mentioned here unless I just missed it would be Fredric Brown.
Names mentioned I would NOT include on that list: Zelazny, Cherryh, Bradbury (though I want to!), Joe Haldeman (an SF favorite, but literature? No.), Spider Robinson, Sheckley, William Tenn(another I want to!), Moorcock, Harry Harrison, Farmer, Ellison (were you serious? Love his essays, can't stand his fiction.), Brunner (though he'd be the closest to making the cut on this list), Clarke, Gibson, Verne, and Adams. Many of these are favorites of mine that I read regularly, but they're in the "light reading" stack, not the literature stack.
This is all just opinion, of course, but I think far too many are getting the nod on this thread; I even think my list is too long. Probably should be trimmed to just LeGuin and maybe Ballard, but I can't do it.