This is always a tough question, because it depends on who its for and how they want to use it.
I would agree that the Unofficial Guide is the most comprehensive and has the most information. You do have to be careful with it as it can easily lead to overplanning.
Birnbaum's is probably the best for a good overview, telling you what is there. But if you need information on park strategy, UG is a better bet.
I'm personally not a big fan of PassPorter. The concept is excellent, and some if it is very well done, but I think the authors made a huge mistake inserting themselves in the book. I'd recommend that one mainly if you need to get yourself organized before you go.
Actually, my favorite Disney book these days is Steve Barrett's Hidden Mickey's, because that has a lot of information I don't know, but would be totally useless for general information.