Very gripping read!
There has been many great suggestions here, and probably a good 15 years of reading material. Some of my suggestions I would like to add are these: (warning, some of these are softer sci-fi, or Science-Fantasy mix)
LeGuin- Don’t forget The Lathe of Heaven ; check out the movie as well, blockbuster generally has it if there are any around where you live. (a great story about a guy that when he dreams he alters reality- he thinks he is going crazy and sees a shrink whom then takes advantage of him to change the world as he (the shrink) sees fit)
Robert Silverburg- So many novels has this guy published, and so many of them are wonderful. I would suggest his Adaptation of Asimov’s work Nightfall. The story of a race of creatures that due to alignment of their suns, has daylight ALL the time, except once every 1000 years, where there is one night of darkness. Will the darkness drive them mad and destroy them? The great unknown? An awesome read.
Also still on Silverburg, and older one of his, but one I really love is Kingdoms of the Wall. Click the link for the story. One of my all time favorite novels (lost my copy though, pretty hard to get ahold of)
You gotta check out Jack L. Chalker’s Well of souls novels, the original series anyhow. (series, btw, 4-5 books per series)
Some more of my favorite Sci-fi stand alone novels-
Ship of Fools- Richard Paul Russo. The story of a group of colonists that ship gets lost. They search for other intelligent life has become their main mission. They soon come to face a enemy of mysterious identity. A dark and fascinating read. I would rate this my #2 favorite. (for writing skills mostly)
An oldy but goodie- Flowers for Algernon by Daniel Keyes. An experiment in which a mentally retarded man is transformed into a supergenius. Written wonderfully from his perspective. Tragic and wonderful. A must read.
How like a God- Brenda W. Clough. Sort of fantasy really, a Man wakes up one day and discovers he is Telepathic. As his powers grow, his sanity weakens. He must search for stablity, and search for others like him, or for the one that made him. Awesome book.
Santiago: Myth of the Far Future- Mike Resnick. An old style western set in space. Mythic prose, a hero searches down a powerful yet nebulous enemy of the law- Santiago. Epic, gripping, and enjoyable. There is another book out now called Santiago returns or something similar.
The Time Ships by Stephen Baxter. A bit more of a hard sci-fi. This takes up after the Time Machine by H.G. Wells. Time travel far into the future. Hard to explain, but definately a great book.
Destiny’s Road- I know somebody warned you against Larry Niven, but this book is great. Jemmy is a boy that sets out on an adventure to find out where the Road goes, and where the ship that made it came from.
Blood Music- Greg Bear. A genetic scientific experement gone awry. A powerful intelligence springs forth and America is destroyed in the process. But is it? A definate addition to any list.
The Boat of a Million Years- Poul Anderson. A group of Immortals make their way through time, to our present, then our future and discover their true potential. Then they take a ship into space to discover what is out there. Great read, especially if you love reading about immortals and like some history.
Legion of the Damned- William C. Dietz. Really an interesting read about the French Foreign legion far in the future when they are cyborgs and giant robots fighting an alien invasion. A glurge book mostly, but wonderfully engaging.
The Icarus Hunt- Timothy Zahn. A renegade star frieghter pilot takes on a job and is chased by some powerful enemies that control 95% of space flight. Jordan (the pilot) soon enough discovers that this is no ordinary cargo, but something that may bring mankind a power that can change the course of human history. Enjoyable read.
Madness Season- C.S Friedman. Daetrin is a strange man. He has survived an alien invasion, survived several centuries of inhabitence, and is probably around 500 years old. When the alien conquerers discover something odd about him, they take him away off planet, where he meets some pretty interesting beings. Can he free humanity?
This Alien Shore by C.S Friedman is also a great read. Jamisia is a girl that apparently knows something, or has something somebody wants. She runs across the galaxy, running from strange but ruthless adversaries, trying to unravel the mystery of her identity and importance. She will uncover a secret buried deep within her psyche a secret which the universe may not be ready to face…
Well, those are a few of my favorite. I think these and the others suggested are all good and will keep you busy for quite some time.