A single book? Impossible. I don't think I could even name my favorite Zelazny or PKD. Many of the works of both are in the running. As is "Dune", for sure. Possibly "Snow Crash". "Protector" might not quite make the cut, but comes somewhat close - from my recollection it was my favorite Niven (not counting short stories)... I'm currently finally getting around to "The Mote In God's Eye". Further back, "Foundation" was pretty amazing, and I've a bit of a soft spot for "Childhood's End". Bester's "The Demolished Man" is well worth a read, but I think also doesn't quite make the cut.