Hitchhikers Guide is a must read for all readers ever, just to get a taste of that style of humour.
When the Lion Feeds - Wilbur Smith
for any Indiana-Jones/African adventure-esque fans.
As part of a childhood type thing, how about Redwall? You haven't had a perfect childhood until you've read at least one Redwall.
and then all the social satires like everyone's said i.e. 1984, Brave new world, Catch 22, Farenheit 451, Utopia,
and the literary giants of speculative and classical fiction - LOTR, The Hobbit, Space Odyssey, Hamlet, The Magus, stuff by Dickens...
and the new-age acclaimed titles - Life of Pi, Fight Club... err that's all that comes to mind right now
Oh and I honestely don't think Dan Brown is really that great.. sure it's really entertaining and a must read, but it really isn't masterful enough. There's just so much hype around his books ever since the Code skyrocketed the sales in Dymocks.