I must say that it's quite nice to see a mixture of both old and new SF titles in this thread. There are a three of my absolute favourites that i can't enjoy as they don't fall into the SF category. My own Science Fiction favourites are:
1. The Player of Games by Iain M. Banks. I've read this books many times and i really like the subtle layers and the digs at our own inadequate society.
2. The Forge of God by Greg Bear. Again, i've read this many times and the ending is just incredible. There are a couple of other Chrons reading this at the moment and i really hope that they enjoy it as it's a massively underrated book in my opinion. The Anvil of Stars was equally enjoyable, but the SF was a lot harder.
3. Necropolis by Dan Abnett. I came across Dan by accident and i have to say that with one or two exceptions i have very much enjoyed every one. Necropolis was my favourite of the Gaunts Ghosts books where i really appreciated Dan's character writing. A hugely exciting book this one. A lot of people tend to look down on it as it's based in the Warhammer 40K universe. Their loss i'd say.
4. Revelation Space by Alastair Reynolds. This book appears to have produced a fair division among the people here in the Chrons, but i very much enjoyed it. I have very much enjoyed all the novels set in the RS universe, but the initial book has to be the best. I can't really get on with his stand alone books for some reason. Don't get me wrong, they are good too, but they're not as good as these. Apparently, Alastair's going to return to teh RS universe after his next trilogy. I for one can't wait.
5. The Hitch Hikers guide to the Galaxy by Douglas Adams. (I always look to the series as one book as i tend to read them in a single collected volume.) Very funny, very insightful and very entertaining. Comedy doesn't lend itself to SF, but this book manages it very well.
As with others on the boards there are so many honourable mentions. Unforunately, there can be only five. (The non SF books that would've been in here were Clive Barkers Weaveworld, It and the Stand by Stephen King.)