Steinbeck is probably the strongest writer on there.

Kerouak is basically Oscar Wilde with worse word-choice. If you're gonna read Kerouac, read Big Sur. After which, stop reading Kerouac and read Oscar Wilde or Edouard Dujardin.

Hunter S. Thompson is a fun and entertaining read, and just phenomenal for people interested in journalism. It's definitely one of the best journalistic works out there, but as far as novels go and exercising your mind... It's not too complex a book. Still, fairly deep, very interesting.

Bradbury is a phenomenal writer, but above him I would recommend Aldous Huxley, particularly Brave New World . He's also a writer of science-fictiony flights of dystopia, but really has an even more entertaining style and imagination than Bradbury.

Kurt Vonnegut is a ****ing genius.

Roald Dahl is actually very interesting, and not, as was suggested, a light read if you really want to look into the depths of his works. Personally, though, I think Steinbeck will give you the strongest, most solid grounding in the world of literature. I voted for The Pearl, but really you should read East of Eden if you're going to tackle one of his truly great novels.