The Nights Dawn Trilogy by Peter F. Hamilton (gargantuan and lengthy space-opera!)

Several books by Philip K. Dick: Martian Time-Slip, Time out of Joint, Ubik, A Maze of Death, A Scanner Darkly, The Man in the High Castle, Eye in the Sky. These are my favourites, but basically I haven't found a book by Dick which is not worth reading at all.

Roadside Picknick by Arkadi and Boris Strugatzki.

The entire Alex Benedict series by Jack McDevitt (unusual setup - an antiques dealer in the future). Eternity Road is worth a reading as well.

The controversial Starship Troopers by Robert Heinlein.

I definitely want to read that one. IMHO, the film was misunderstood by a lot of movie audiences.

Some pulpy but interesting novels would be the two books by David Weber and Steve White called "In Death Ground" and "The Shiva Option". They're good space battle/tactician books mixed with the political side of things. They are based on the "Starfire" tabletop gaming universe which I've never played but the books are a good read.

Dune - Frank Herbert
2001 - Arthur C. Clarke
Stranger in a Strange Land - Robert A. Heinlein
I Robot - Isaac Asimov
Starship Troopers - Robert A. Heinlein

There are more, but these certainly form a strong list.

Very nice list with some great suggestions I need to check out.

I've been almost totally and exclusively reading Clarke for my Sci-Fi fix, but of course I'm now trying to find other "hard" SF authors.

I read the Nights Dawn Trilogy by Peter F. Hamilton a couple of years back and concluded that this brand of Sci-Fi was not for me. I ended up giving those books away to reclaim some space in my bookshelf.

Some of the authors I have found to enjoy are;

Gregory Benford's Timescape (I'm thinking of gettig more of his books)
Stephen Baxter's Titan, Voyage, Moonseed, Flux
W.A. Harbinson's Project Saucer Series (not recommended if you're prone to depression)
Buzz Aldrin & John Barnes' Encounter with Tiber

Starship Troopers, Heinlein
Dune, Herbert
The Legacy of Heorot, Barnes, Niven, & Pournelle
Moving Mars, Greg Bear
Any of the Man-Kzin Wars
Psychohistorical Crisis, Donald Kingsbury

ETA The Worthing Saga, by Orson Scott Card

The problem is the movie had absolutely no relationship to the book other than the title, the main character's name, and the fact that "bugs" from Klendathu were the enemy. I wouldn't even say the movie "strayed" from the book, it was more like Verhoeven's screenplay writer wrote a completely different movie and said "hey, I liked that Heinlein book, I'll use the title, main character's name, and the enemy for my screenplay."

• Foundation trilogy - Asimov - but actually I like most of Asimov's stories - even the dated ones like the early Elijah Baley stories.
• Rama series - Clarke
• De sluimerende stranden van de geest - Eddy C.Bertin (only in Dutch I'm afraid... too bad because it's great horror SciFi)
• H2G2 - Douglas Adams
• Tschai -Jack Vance (but it's quite a while since I last read it... maybe I don't like it anymore)

...and a very honorable mention of Chris Foss... when I just had discovered Science Fiction I chose many of the books I got in the library based on his artwork.

I still need to read "Rama" from Clarke and the "I Robot" series from Asimov. One day..one day...

I really liked the first Rama novel, but didn't like the second one very much, I think because Clarke was assisted by Gentry Lee, and the style and some story elements were very different from what Clarke would normally do. Kind of soap-operaish, and just not what I read a Clarke novel for. I didn't read the following novel.