Surely You're Joking, Mr Feynman? by Richard Feynman. One of the most brilliant physicists of the 20th century, he was also one of its greatest characters. This is part of his autobiography
My vote goes to this as well, assuming we aren't restricted to popular science books by Hawwking and Greene...
When reading Feynman's books you really get a feeling that he's reading out loud to you. I've always found a lot of the pop science books painful to read in places, particularly Hawking's.