So difficult... Any of George Orwell's novels would be up there with ease. It's quite upsetting how overshadowed his social novels have become, by his political ones, but all of them are utterly brilliant.
Something by Christopher Hitchens would be up there as well, but with a collection as rich as his, it would be difficult to choose. But I would go with Letters to a Young Contrarian or the Trial of Henry Kissinger. Or the Missionary Position, because of how deliciously provocative it is.
In Search of Schrödinger's Cat by John Gribbin was incredibly illuminating for me, and I had a thoroughly enjoyable time reading it.
Dreams From My Father by Barack Obama is also a spectacular book. I can only wish that I read it before he became president.
And how could I forget, The Republic of Fear by Samir Al-Khalil. Anybody that questions the moral case for ousting Saddam Hussein's regime from its ownership of Iraq needs to spend some time with a few chapters of this book.