I tend much more towards Robert A. Heinlein. Asimov was great, but I can't reread his stories all that well. Heinlein, on the other hand, put such rich detail and magnificent story lines in his work, that I have read many of them over and over.
David Weber is my favorite current author. His books are very much oriented towards military, in fact his Honor Harrington books are very similar (on purpose) to the Horatio Hornblower books of historical fiction.
All of the above are truely great, Asimov and Heinlein espcaily. But to me, the best that I have ever read would have to be Piers Anthony and Anna McCaffrey. Anthony because go into amazing detail, and he uses abstract concpets to make everything better, IE the Tarot series. McCaffrey is great because of the Dragon Riders of Pern. you don't truely relize that its Sci-Fi till you are already wrapped into the books.
I really liked Ray Bradbury, he had an interesting way of making the most mundane tasks appear to be very romantic and heroic.
I also liked Asimov a lot, he had some unique ideas, and did a good job of pulling them off. Like his 'The machine that won the war" was classic
But I'd have to say that my favorite is Douglas Adams. He does the funniest things to english, and his characters are all so bizarre, yet so true to life. I just can't get enough of his stuff.