Welcome to my list of Top 10 novels!

1. Dune (Dune series)- Frank Herbert - While this is my all time favorite novel, it's not for everyone. Herbert has created an incredibly detailed universe here, and it's very easy to get lost in all of it's terminology the first time through. While diehards would call the sacreligous, I would recommend watching one of the movie versions first (I personally prefer the 1983 David Lynch version, but the 2000 Sci-Fi channel miniseries is also good.) In any case, the novel itself is so full of fascinating characters, inventions, worlds, and ideas that it will always stand alone in the science fiction field.
2. Ender's Game (Ender series)- Orson Scott Card - Card is one of my favorite authors period, and I'm just picking this novel since it is his most popular, easiest to find, and a good introduction to the kind of stories the man writes. It's the tale of a very young child handpicked by Earth's space fleet to undergo rigorous training to become the genius needed to defeat an alien threat. Card is a master at creating involving characters and placing them into fantastically involving situations... you can pick up anything he's written and be assured of a fantastic time. I look forward to each new novel he writes, and thankfully he's qutie prolific!
3. The Sword of Shannara (Shannara series)- Terry Brooks - Brooks is all too often written off as a hack fantasy writer, which is unfortunate. While it's quite true that the Shannara world is rather Tolkein-esque, the skill in which the tale is told makes it all seem new and exciting. This first book is fantastic, and the sequel, Elfstones of Shannara, is even better. Also well worth reading are his Knight of the Word books... excellent dark modern fantasy.
4. Foundation (Foundation series) - Issac Asimov - Absoluetly required reading!. This extraordinary tale of the fall of a Galactic Empire provided everyone who followed with the ideas that have shaped science fiction every since.
5. Hitchhiker's Guide to the Galaxy (Hitchhiker's Guide to the Galaxy series) - Douglas Adams - Hilarious novels with a science fiction setting. All five books in this series are light, funny, and very easy to read. It's tremendously sad that their author died recently.
6. The Moon is a Harsh Mistress (gasp, no series!)- Robert Heinlein - While almost any Heinlein book is a lot of fun to read, many (ok, most) of them are full of some really strange ideas and characters that make taking the book seriously difficult. This book is the exception. It tells the tale of oppressed Lunar colonists who attempt a revolution against the Earth. Full of interesting ideas and characters, this is Heinlien's best work.
7. Dragons of Autumn Twilight (Dragonlance series) - Margaret Weis and Tracy Hickman - Since these are set in a Dungeons and Dragons gameworld, it would be easy to dismiss these books as worthless spinoffs. And the fact is that pretty much every DL book NOT written by Weis and Hickman IS, in fact, pretty awful. But the original six books are absolutely superb fantasy with characters that seem more alive and interesting than ANY other series I've read yet.
8. The Lion, the Witch, and the Wardrobe (Narnia series) - C.S.Lewis - Yes, these are children's books. And yes, they are Christian allegories - but veiled well enough that many readers have never noticed that. In any case, these were my favorite books as a kid, and I still enjoy them. An excellent introduction to fantasy for young readers.
9. Childhood's End - Arthur C. Clarke - Clarke is rightfully honored as being one of the greats in the field of science fiction, yet many of his books are lean heavily toward very "hard" science-fiction. Which is great for fans of that field, but not so much for those of us who just enjoy a good story. Childhood's End, on the other hand, is a great, powerful story. The initial setup may seem very familar to fans of V and Independence Day (both of which this novel predates by quite a bit!)
10. Carrion Comfort - Dan Simmons - I actually have read a great deal of horror books, many by the big name authors like King, Barker, and Rice, but this novel is my favorite of the genre. He takes a genuninely scary idea and develops it incredibly well into an epic story with great characters. While this is probably the least-known book on my list, it is well worth looking for.

Honorable mentions:

Brave New World - Aldous Huxley

The Dark Tower - Stephen King

Wizard's First Rule - Terry Goodkind

Lord of the Rings - J.R.R. Tolkein

Interview with the Vampire - Anne Rice

Also, I've compiled a list of my favorite Star Wars books and comics.