OP did say "books", not "book". I'm surprised how many posts point to a single book. The plural makes it easier. 'e also didn't specify criteria for "best", which makes it kind of open. So:
1984 - Not the most enjoyable book I've read by a long shot. It's grim. But I don't see SF as purely entertainment. And I can't think of any other book that so exemplifies this higher purpose. This is essential reading for every human being. Soon, it may be too late. It addresses the question "How bad will this get if we don't stop it?" and the answer is "pretty damned horrible".
Little Brother by Cory Doctorow - Doctorow has the right idea in carrying forward Orwell's theme in "YA" books like "Little Brother", which, unlike Orwell's, is a very enjoyable book. It is the kind of thing Heinlein wrote so much of, what Niven called "a dystopia with hope", carrying the message that the future doesn't just happen - it is MADE by our actions or lack thereof. It is definitely enjoyable by adults. He gives it away like shareware (in other words, pay what and if you want to) on his website in electronic formats. Paper copies are in print also, of course. This, and it's 2 sequels, are certainly the best current YA SF I'm aware of and I think the category is of profound social importance. I suggest giving copies to any kid bright enough to read.
Ringworld - Still probably the most enjoyable single SF book I've read. Overall, NIven is my favorite writer. I don't agree with those who call him "the Dean of Hard SF" - he makes too much use of psi powers, genetic luck and such to classify him thus. That title I might give to Hal Clement. Niven seems to harden up when he has coauthors though. Maybe they tend to keep him honest. Which brings me to:
Lucifer's Hammer - Best apocalyptic disaster novel ever. And written BEFORE Alverez told the paleontologists and geologist they had it all wrong about K/T.
Trader to the Stars by Poul Anderson - I think this may have been republished later with some more stories of van Rijn in a bigger book with a different title. Anyway, all the van Rijn stories, especially The Man Who Counts, whatever title they are grouped under.
Fallen Angels - Best politically incorrect novel ever. Also a global disaster novel. Niven and Pournelle, with various friends, have done a great job of nearly ending the world half a dozen times. These guys are DANGEROUS.
The Whenabouts of Burr by Michael Kurland - Maybe the best pseudo-time travel novel I know. Some people say this is the anscestor of most parallel time track stories. Well, maybe. It certainly was NOT the first. I'm pretty sure H. Beam Piper did some similar things in the 50s. But this, which was mid-70s, is much more vivid than Piper's tales, and it may be true that he got a lot of writers thinking about it.
I'm not a big fantasy fan. Tolkein bored the heck out of me. But I did like these:
The Unbeheaded King trilogy - Let this stand for any or all of de Camp's fantasy. They are wonderfully funny.
Silverlock by John Meyers Meyers - It is an old cliche to say something can be read and appreciated on several different levels. I don't know of any other book that this is AS true of. I've read it at least 5 times and gotten a few things I had missed on earlier readings every time. And I don't just mean literary references. I'm tempted to give examples, but no, no hints, no spoilers. Definitely, best fantasy, open category.
