Books I could not do without; Ringworld (Larry Niven) The Forge of God (Greg Bear) Space (Stephen Baxter) but my favourite SF book of all time, and one which influenced all the others; Last and First Men (Olaf Stapledon)
How can i pick just one? American Gods and everything else by Neil Gaiman is just incredible. (Actually i think some of his Sandman books are better even better than AG) Every Foundation book ever except Foundation's Fear, I like Gregory Benford but that book sucked. Currently reading the Uplift books by David Brin. A very interesting series.
Startide Rising by David Brin. Nice, broke some new ground. Lord of Light by Roger Zelazny would be worth mentioning if we weren't limited to only 1 selection. ;-)