I vote for either the Unofficial Guide or The Passporter. I have used both of these and like them both for different reasons. The Unofficial Guide is very straight forward. They have no link to Disney. When I wan't an honest opinion, I want it from someone whose bread isn't buttered by Disney. Birnbaum is a good guide book, but a previous poster put it best. It might as well be coming from WDW marketing. The Passporter has a lot of useful and very well organized information. I used this on several of our trips and it was nice to have the phone numbers handy as well as the pockets to store stuff like hotel confimations, tickets, etc. And I agree that they have the best maps of all the guidebooks.
I think for a first time visiter, I would go with Passporter because it gives the information without being too overwhelming and it has the features that can assist in staying organized. Although, I read the Unofficial Guide before I started our very first trip, and it wasn't overwhelming at all.
Either way, I wouldn't limit them to just one source. Buy one and include a list of the other resources that they can check them out on their own.