+1 on Dan Simmons' Hyperion Saga
+1 on John Steakley's Armor
+1 on Alfred Bester's The Stars My Destination; also check out The Demolished Man
+1 on anything by Heinlein, particularly Time Enough for Love
+1 on Clifford Simak's City, but I found Way Station was better.
Just about anything by Alastair Reynolds or Iain Banks.
Stephen Baxter's Vacuum Diagrams and Raft
Orson Scott Card's Ender's Game is a classic, and deservedly so.
Bruce Sterling's Schismatrix Plus has some refreshingly new ideas.
John Scalzi's Old Man's War is fast-paced, but still has some substance.
M. John Harrison's Light was an odd little number.
Richard Paul Russo's The Rosetta Codex was good, as was his cyberpunk crime series collected in Carlucci 3-in-1.
For cyberpunk, of course, William Gibson's early work is still the best.
While they're not hard SciFi, Ian McDonald's books are superb.
If you find yourself in the mood for something a little other, check out anything by China Mieville or Jeff VanderMeer.
Tim Powers is phenomenal (non-SciFi) writer whose books follow this premise: "I made it an ironclad rule that I could not change or disregard any of the recorded facts, nor rearrange any days of the calendar - and then I tried to figure out what momentous but unrecorded fact could explain them all." — Timothy Thomas Powers
I can't recommend him highly enough.
Finally, and again off the hard-SciFi path, but I feel worth mentioning is Stephen King's Dark Tower series. While it contains elements of horror, fantasy, and western it is also set largely on a world that is very much past its highly technological prime. (M. John Harrison's Viriconium takes a similar path, but tends to drag.)
Enjoy!