Personally, there are a few authors that will get me to check out a book in depth without a cursory examination:
Lois McMaster Bujold: A lot of people here mention her, and for good reason. It's hardly even sci-fi in parts - more romance or period political thriller. But I don't care, it's awesome. Miles Vorkosigan frequently makes me laugh aloud.
David Weber: Another favourite, this stuff is great. I'll never look at space combat the same way again - although you need a decent grasp of 'mechanical' physics to truly understand why all this stuff works. Bleek.
Oh, and he's done some fantasy too. The main character is a paladin with an arbalest - a small ballista - that he carries around with him.
David Feintuch: I haven't read much of the Midshipman's Hope series, but the super-Christian outlook of the main character makes for some interesting problems. The rest of it's pretty interesting, too. Military scifi of a kind, with a truly weird enemy.
I haven't read any of the Reality Dysfunction, but Hamilton's Mindstar Rising stuff is also cool. That's early to late 21st century corporate/scifi stuff, in which England has become a tropical swamp due to global warming.
And Zindell's Requiem For Homo Sapiens made a mark on me, too. Spaceship pilots fight with mathematics, there are Gods the size of solar systems, and cavemen.
That's all I can think of for now...
PS: Hong, have you tried Histories by Herodotus? It's not scifi, it's fantasy, but I thought I'd mention it. The Oracle (a recurring plot device) is often bailed out on technicalities that the author just seems to make up, but some of the creatures are pretty weird and the battle scenes are top-notch. And talk about world-building; half the book is devoted to setting the scene! Although, the author isn't too clear about some things, and I suspect large parts may have been cut and pasted from some kind of half-baked design document - there's one empire he mentions that 'might not even exist'. Also, characters are handled at arms length; you rarely see what they're thinking.
But Herodotus uses more footnotes than Pratchett, which must be good.
Overall, I enjoyed it despite the inconsistencies.
I'd give a review of Gibbon's Decline And Fall, but I haven't finished reading that yet. The foreshadowing's pretty heavy so far, though. I think the empire might fall, but that's just a guess.