"Off to be the wizard" is pretty light and fun. "John dies at the end" was fun, not really sci-fi, more buffy the vampire slayer esque. "Existence" by David Brin made me want to puke it had so many good ideas. A tiny bit dated, but very smart. "The Martian" I expect you've read it. Good romp, easy to read. Reminded me of a Michael Crichton. "World War Z" was good.

I'm a huge fan of John Dies At The End. It's really not for everyone but I found it incredibly entertaining. The follow-up, This Book Is Full of Spiders is also worth reading.

Anathem and Cryptonomicon are both must-reads by Stephenson too.

The two books I've read by Stephenson and really enjoyed. Never really got into the Baroque cycle because it's so long winded, but that should also be good.

You can try the Foundation series by Isaac Asimov if you like Sci-Fi.

Twice-recommended, I'll check it out - thanks :)

Neuromancer by William Gibson. The Forever War by Joe Haldeman. Rendezvous with Rama by Arthur C Clarke. A Fire Upon the Deep and A Deepness in the Sky, both by Vernor Vinge.

Also, the Culture series by Iain M. Banks, but I would recommend not starting with the first published book, Consider Phlebas. YMMV, but I personally found it hard to get in to and almost didn't continue the series. Later I read lots of reviews of the second book, The Player of Games, and decided to give it a chance and have since read most of the remaining books. I have now gone back and re-read Consider Phlebas and love it now that I have a better handle on the setting, universe, and concepts from the series, but I definitely feel that The Player of Games is a much better intro into the series.

Not for every reader but Stephenson's Baroque Cycle (Quicksilver, The Confusion, The System of the World) is something between historical fiction and science fiction. Worth a try in your case.

Also, it's hard to go wrong with the yearly Hugo Award winners and nominees. Sometimes they are middle entries in long running series, which is worth checking on Wikipedia if you like to start at Book 1. Usually it is just going to mean the whole series is great.

How about a few great authors who have all won?

  Ursula K LeGuin
  Lois McMaster Bujold
  CJ Cherryh
  Connie Willis
  Jo Walton

Isaac Asimov is a must read for people who love Sci-Fi. Some of his books I loved: The Robot Series (The Caves of Steel, The Robots of Dawn etc.), The Foundation Series, Nemesis, the list goes on ...

R.A. Lafferty's literary sci-fi has many historical and non-fiction elements. Out of print but there's a complete collection online called "Talled Tales". Some stories can be read on the web, http://www.ralafferty.org/works/collections/online-stories/

Flatland is great.

Try Ian M Banks if you haven't already for some cerebral sci-fi, also Vernor Vinge.