What a great topic for a thread. I have always loved Sci-Fi and Fantasy, although lately, I have found that all I ever read is related to the Work. However, reading many of my favorite authors listed above definitely evokes a sense of nostalgia :).
So many fond memories of immersion into imaginary worlds: the Hyperion Series (I preferred book 1 and 3 but those were very powerful, indeed).
The Gap series (and if you like Donaldson, don't cheat yourself to his fantasy work, The First Chronicles of Thomas Covenant, The Second Chronicles of Thomas Coventant. He is currently working on the final trilogy of the Thomas Covenant series).
Enders Game and Speaker For The Dead were outstanding. It should be no surprise that this is one of the few times that an author (Card) has won both the Hugo and Nebula Awards during the same year. Also I believe it was the only time that sequels have won the award. Richly deserved. The two last books to the series were weaker in my opinion. Also Ender's Shadow about Bean and Shadow of the Hegemon were quite worth reading.
I really like many of the Culture novels by Iain M. Banks. In my mind Excession and The Player of Games stand out as the best.
I highly second and third the recommendations of the Reality Dysfunction series (Hamilton) and Alastair Reynolds (Revelation Space, ect.) as well as Vernon Vinge.
When we're speaking Asimov it all has literary interest as he was one of the founding fathers of Sci-Fi who you can't skip. I highly recommend the Foundation Series although it's many years since I've read it.
However, I find that my two current absolute favorite authors have not been mentioned and that has to be remedied! They are genre-transcenders and therefore not classical Sci-Fi or Fantasy but just phenomenally good. Firstly do yourself the favor of reading China Mieville's "Perdido Street Station" and "The Scar". He is imaginative beyond description and his style and language is hard to describe. It is as a sensory overload where you're left with your jaw hanging at your chest, thinking "what just happened". I believe that those are his best novels to date. His latest "Kraken" is also good but in a completely different setting and in my opinion, not quite the tour de force of the others.
If you haven't read any Neal Stephenson, you're in for a treat. He just keeps getting better and better. His book Snowcrash is a genre-definer and just as seminal and important as Gibson's NeuroMancer - just better written. The Diamond Age is probably a bit outdated by now. Cryptonomicron is a great read but not really Sci-Fi at all. It'd be more accurate to label it near-fiction and the trilogy called the Baroque Cycle is nothing less than breathtaking in scope and an accomplishment that rivals Tolstoy in depth and quality. It is about the development of natural science, finance, the European Royal houses, adventure, alchemy from the 1600s to the 1800s. It's massive but well worth the read.
Lastly, I can not recommend his latest book Anathem enough. It is about a sort of what-if world that resembles Earth and is very interesting with many mathematical, philosophical and spiritual aspects that I found very rewarding while on my own Path.
Check out China Mielvill and Neal Stephenson and you won't be disappointed :)