So far it's been World War Z, hands down.
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.
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.