If you're into SciFi then you should start with the classics.
Robot series and Foundation series by Asimov are on the "must read" list and will take you at least a few months to go through.
The Hitchhiker's Guide to the Galaxy is there as well. It's not so much a Science Fiction as a comedy in a space setting (pretty much no science whatsoever, but great fun to read).
Then there is Arthur C. Clarke with the Space Odyssey, Randez-vous with Rama and pretty much anything else he wrote.
Robert A. Heinlen - Moon is a harsh mistress, Stranger in a strange land (both are a must read for every SciFi lover), everything else by him.
1984 by George Orwell (actually called Nineteen Eighty-Four). There are so many references in the every day life to this novel that if you haven't read it you are missing out a lot.
Fantasy... Well you can't go wrong with ASOIAF already mentioned. It is not a finished series, though.
I too should get back to some reading...
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