Dune, 50 years on
I thoroughly enjoyed Dune.  It had an interesting story, set in an even more interesting world.However, it's also the kind of book that should never have had a sequel; other books in the series not only don't measure up, but actively undermine what made the original interesting.  It's possible that other good stories could have been written in that universe, but they weren't.  I highly recommend reading only the original.