The truth is, it depends on what you want. There is far more usuable information on saving money, line strategies, etc in the Unofficial Guide. It can be really overwhelming though. It contains a lot of humor and the author is not shy about saying if something is a poor attraction or not worth the wait. The Official Guide contains lots of pics and is a much easier read, but is less of a critique and reads more like a press release. Still quite useful though. The passporter's guide is terrific too, and contains tools that the others don't have. I actually purchase the Unofficial Guide every year, and the Official Guide every couple of years. I haven't purchased the Passporter's guide yet, but I know people who love it.
I have one other recommendation. Pick up the Field Guide to Hidden Mickey's in addition to any other guide book that you decide to use. You won't regret it!