Ursula LeGuin is an incredible author, but might not sit well. I'd check the Wikipedia summaries for a few of her books--maybe Changing Planes, definitely Lathe of Heaven and The Left Hand of Darkness (two different books, just in case you read that as one title), probably Birthday of the World. Or whatever else jumps out at you. The Earthsea cycle is great, but is less sci-fi and more straight fantasy. Her sci-fi stuff doesn't always follow the typical plot structure--there doesn't always feel like a climax, and relatively little of it is revolves around some big crisis that has to be averted. But it's still excellent stuff.
Failing that, Arthur C. Clarke is always a good read, pretty much no matter what.
Michael Crichton (Jurassic Park, Andromeda Strain, Sphere, etc) is a good author. Not quite as good as some, but it's always a fun read and is very well written.
I'd recommend LeGuin most highly, though, if you don't already know her and if some of her stories sound interesting to you.
Someone earlier in this threat said Lovecraft. I'd definitely recommend Lovecraft, but you specifically wanted sci-fi. Lovecraft wrote a bunch of really influential horror stories in the early 1900s (1920s to late 1930s if I have my dates right) and gave us Cthulhu. A lot of his stories deal with themes of the unknown/unknowable, the limitation of human knowledge, and our cosmic insignificance. BUT you can get just about everything of his through Project Gutenberg, as it's out of copyright, so don't bother getting for free what is already free.