Each guidebook has its own strengths and its own weaknesses.
Unofficial Guide: Completely unbiased information. If the folks at the UG like something, but they get complaints about it, the complaints are posted right next to their praise. And vice versa if the UG folks DON'T like something.

It's strength also lie in its touring plans. They work. I also love the road maps in the UG. The UG's weaknesses? I'm not a big fan of the park and resort maps inside. And while they update their information on their website, and I just prefer the layout for Passporter updates. But that's highly individual, and it doesn't detract from the guidebook.
Birnbaum's: AWESOME photographs! I love the way this guide is laid out. Its weakness is that the information is very much one-sided, and not very detailed. They give, you as the UG says, simply the 'Official Line'. Having said all that, there's no bigger treat for me than to buy a copy when the updated version comes out each year.
Passporter: THE PASSPOCKETS! I haven't used them yet, but this is the Passporter's niche. It helps keep everything organized, and the pages in the book itself are wonderful - where you can keep track of all your transportation arrangements, resort reservations, and dining arrangements. I LOVE the park maps in this one. Weaknesses? The information feels (to me) to be very pixie-dusted. There's nothing necessarily wrong with that, but I really like to hear what bad things I might expect alongside whatever wonderful things there are. Once again though, that's just my personal preference.
Those are the big three. I've also bought a few more: The couple's guide to Disney World by the Perlmutters, Frommer's guide to Disney World, I'm not at home, but I think the other one I have is called the Economist's Guide to Disney World? It's all about saving money. It's white with Mission Space on the cover. I also have the Unofficial Guide to Universal Studios, Hidden Mickey's, and Universal Orlando: The Ultimate Guide to the Ultimate Theme Park. Although it's not a guidebook, I also have the Disney World Trivia Book by Louis Mangello.
Having bought ALL of those, if I still had to choose just one, my money's still on the Unofficial Guide. It has the best information. The rest is just decoration.
