The Algebraist

Of course I love the Culture, but it says a lot that Banks' non-Culture based sci-fi is also fantastic. The Algebraist uses 'hard' sci-fi amazingly to create a fantastic, connected yet contained world, and tells a wonderful story within it. The worldbuilding is rich and complete, the characters are all marvellous, and I freely admit that I actually gasped out loud when everything in the plot came together for me at the end. It's not quite a Look to Windward, but it's marvellous nonetheless.