tsuwm's right (as is often the case!) - this is so subjective as to be practically unanswerable, and I also agree that the best way is to read - or at least attempt to read- anything you can lay your hands on.
My own criterion for a "really good" book is that it has to be one that I want to read for a second time. Very few come into that category. When I was very young, the "Just William" books were favourite and recurrent reading (I wonder how they would go down with an American - not too well, I guess, as they rely very much on an English view of the world - and a middle class one, at that!) Now, it is "The Lord of the Rings" (JRR Tolkien) and some of Dylan Thomas' short stories - "Adventures in the Skin Trade" are good. Lord Macaulay's poetry is always re-readable, for me, as is Omar Khayyam. Most (not all) of Dickens, with its complicated sub-plots, I can read again and find that I have forgotten half of it.
The only Shakespeare that I re-read are his sonnets - and those not often!
But there is so much that is interesting, funny, touching on its first and only reading that I will try just about anything - even Mills and Boone!
