Robin Hobb- Read the Liveship Trilogy

The Farseer & Fool trilogies are set in the same world, but aren't quite as good. The Soldier Son trilogy is also well worth a read, but unfortunately isn't in the same leauge unfortunately.
Another much overlooked choice is the Saga of the Exiles by Julian May. Sadly these have been out of print for several years now, but they form one of the best fantasy/ SF clones that I've come across.
SF-wise, the two must read authors are Isaac Asimov & Ben Bova.
I also agree with the Arthur C. Clarke reccomendation, but some of his works are better than others
I really wouldn't bother with Umberto Eco tbh, somone who needs all of 5 pages to describe the contents of a rucksack (Without any side stories either) should be shot!
You name it, I've read it, so my opinion may be a little biased

In all seriousness though, you've got to decide on amongst other things the style of book you're looking to read- If you're looking for fast-paced action, then the field of authors is much different to those who take a more philosophical and moral slant to thier work. (On that tack, I'd reccomend Harry Harrison & Jack L. Chalker; Both may be slightly harder to find, but have produced some exceedingly good stuff)
Cheers
-Leezer-