So far it's been World War Z, hands down.<p>The book lends itself to the audio format extremely well due to the fact that a large part of the book takes place in the form of interviews. The author (Max Brooks) plays the part of the interviewer.<p>Another reason I love it is the cast. Alan Alda, Rob Reiner, Henry Rollins, Mark Hamill, John Tuturro among others make it a fantastic listen.
The tenth anniversary edition of American Gods is read by a great cast, and is more like a play than an audio book. Each character is read well. Really sucked me in.