When I was younger the answer to this question would probably have been Isaac Asimov's Caves of Steel especially the Panther paperback with the great Chris Foss artwork on the cover. A few years ago I might have chosen a rather strange and gloomy book called Shipwreck by Charles Logan. But right now my choice comes down to two books both of which I have blogged about before.
Pattern Recognition by William Gibson is a fascinating novel about a world that is only very slightly removed from our own. It has a likeable and quirky heroine and an intriguing plot about a strange corner of the internet. It also reduced the remarkable character of Hubertus Bigend who links this book to the next two in the Blue Ant trilogy.
On the other hand Consider Phlebus by Iain M. Banks is as far from our world as you can get. It was also the first book in a series which introduced us to the Culture, but like the best of the Culture novels it is about an outsider and a huge space opera conflict. It has grim, militaristic aliens, some fun bits of space piracy in the style of the old role playing game Traveller, there's a sexy Culture agent, and of course our first meeting with Culture AI Minds and their hilarious ship names.
It's tough to choose between them but because Pattern Recognition is more of a contemporary novel than true science fiction I am going to pick Consider Phlebus as my favourite science fiction novel.