Second for Antifragile. His earlier books are more approachable, but I found Antifragile fascinating and I think about various principles from the book regularly. One of a handful of books I keep on the bookshelf at my office at all times.
Agree agree agree. He's bidding for the sort of intellectual immortality with Antigragile. The Fat Tony dialogues are really a pleasure, and a throwback to Plato's Dialogues. Really enjoyable, meaty stuff.
