I too cannot pick a single book, however a selection series I thoroughly loved reading:
The Dreamers, by David & Leigh Eddings
Dune, by Frank Herbert (and to a lesser extend, the sequels/prequels by his son)
Discworld, by Terry Pratchett (mainly stories about The Watch)
If I was going to pick something from Eddings it would probably be the Elenium; didn't really enjoy the Dreamers. Dune is brilliant and I can't think of anything else in the genre that stands up to it, sadly including its sequels. Big fan of Terry Pratchett, read most of the Discworld novels.
