Besides asking me for my personal SF recommendations, sometimes people ask if there is anywhere where there are lists of good SF books to consider reading. Is there ever! Look no further than the very nicely-produced Worlds Without End site. There's an entire section devoted to elaborately displayed lists (all those beautiful covers!) of SF books from various sources, including all of the best ones available as far I can figure out. I particularly recommend the Classics of Science Fiction list, based on a meticulous synthesis of critical opinion, and the SF Masterworks list which collects all the books issued by the British firm, Gollancz, in that fantastic series. But there are lots of other good lists, not to mention an entire other section that has complete book lists for all the major SF awards.
Fabulous resource. Highly recommended.