I love the Webmage series by Kelly McCullough -- don't be turned off by how bizarre they sound when summarized...it is one of the best series I have ever read. (And I don't say that lightly).
Also, pretty much anything by Heinlein (I know he has been mentioned earlier in this thread). Especially Starship Troopers and Stranger in a Strange Land.
I am seconding (fourthing?) Patrick Rothfuss' Name of the Wind (first in the trilogy). I almost hate to use the word epic because gets so overused in the fantasy genre, but this is one to which the word truly applies. I've read a lot of fantasy and most seem to follow a specific formula. This seems bent on twisting the formula so perfectly you don't even notice it has been done until later. It is that good.
I also echo the sentiments of others who recommend Orson Scott Card's Ender's Game. And the entire series (while some are better than others) is fairly well-written. Definitely not a waste of time.
Someone else mentioned C.S. Lewis' sci-fi series Space Trilogy, beginning with Out of the Silent Planet. It also breaks quite a few formulaic 'rules' for writing sci-fi (which is quite funny, considering the first book was written in 1938).
And I didn't see this one on the list, so please pardon me if I am (again) echoing others recommendations- Game of Thrones by George R.R. Martin from the A Song of Ice and Fire series. I know it's been generating some press lately because of the HBO series it has spawned, but it really does deserve the hype. It is, as many have already called it, "a cross between The Sopranos and The Lord of the Rings." There's action and adventure and high drama and swords. :D It is really quite good and there are several in the series.Happy Reading!
Anything by China Mieville is amazing. I'd highly recommend starting with Perdido Street Station and then move on to the rest. He has a B.A. in Anthropology and PhD with Distinction in International Relations, which is a really interesting educational background for a Sci Fi writer. The world he creates is incredibly well fleshed out and he goes deeply into the social interactions of all the various species that exist. However, he doesn't let the book get bogged down in it, he still tells a fast-paced, interesting story.
Also, I may be a bit partial, because I've been good friends with the author's son for years, but Rite of Passage by Alexei Panshin is excellent. It beat Do Android Dream of Electric Sheep by Philip K. Dick (another great book) for the Nebula in 1968. It is a fantastic book.
Also, if you like steampunk-ish stuff, there is an anthology called Steampunk by Jeff and Ann VanderMeer that is incredible and gives you a good jumping off point for the genre. And I'm in the midst of reading a book called "The Glass Books of the Dream Eaters, vol. 1" by Gordon Dahlquist that is pretty excellent.
I'm not sure if anyone's mentioned them, but I found Sword of Truth series by Terry Goodkind to be wonderful.
I can agree with others that Dune is nice, but my experience is that I've only read the first book of the series. Also, I'm n-thing the George R.R. Martin's "A Song of Ice and Fire", and I too wish him good health, long life and great productivity. :)
The first SF/Fantasy books I've ever read were by John Varley. It was a trilogy and the individual books were named Titan, Wizard and Demon. I was a kid when I read those so I don't remember much, but I remember they were quite nice.
There's Harry Potter and The Lord of the Rings, of course, not sure whether they need to be mentioned here.
And let's not forget the Isaac Asimov classics.
Right now I have several books on my reading list, all SF which had relatively good reviews:
Larry Niven & Jerry Pournelle, "Lucifer's Hammer"
Dan Simmons, "Hyperion" and "The Fall of Hyperion"
Valmore Daniels, "Forbidden the Stars"
Kevin Bohacz, "Immortality"