I'm wary of labels such as "must read" - but a couple of books I'm glad I read:

"Dahlgren" by Samuel Delaney (and most everything else I've read by him as well).

"Earthsea" by Ursula LeGuin - and "Powers".

"Diamond Age" by Neal Stephenson.

"Holy Fire" by Bruce Sterling (I also enjoyed Islands in the Net, Zenith Angle and Schismatrix).

"Foundation" trilogy + prequel by Isaac Asimov.

"Brave New World", Aldous Huxley.

"Deepness in the Sky" / "Fire Upon the Deep" by Vernor Vinge.

"Something wicked this way comes" by Ray Bradbury.

And a couple of comics/visual novels:

"V for Vendetta" and "Watchmen" by Alan Moore (David Lloyd/ Tony Weare and Dave Gibbons / John Higgins).

Dave Sims: "Cerebus" (I don't think I've finished this yet, but the first ten volumes or so is.. Something else).