First off, I think we all know that choices for 'the best book ever' are most definitely going to vary, so there's no point in even mentioning its subjectivity.
Many of my favorite books have already been mentioned (wayyyy to go iamcolin for mentioning Hitchhiker's Guide & Thalion for Camus' <i>The Plague</i>) but I am stunned that none of these have been mentioned:
Brave New World by Aldous Huxley
Animal Farm by George Orwell
A Tale of Two Cities by Charles Dickens
(and lets not overlook the simple, yet wonderful beauty that is children's literature)
The Phantom Tollbooth by Norton Juster
The True Confessions of Charlotte Doyle by Avi
The Wanderer & Walk Two Moons by Sharon Creech
Just to name a few...