The Official Guide for GMAT Review is considered by most people to be a must-have. It's written by the people who run the GMAT, and contains over 1,400 questions. My advice is to use other prep materials/services first, and then start going through the entire book 2-3 months before you take the actual exam. I recommend doing it this way because you ideally want to already be in pretty good shape by the time you practice on the real questions, otherwise you're sort of "wasting" them.
I myself just used a Kaplan book (that came with a pretty good CD) and then
the Official Guide. If you're considering prep courses, also consider Veritas. They're considered a little more high-end than PR or Kaplan, and people seem to be very satisfied with their classes. Also, Integrated Learning is another small one to consider. They do one-on-one tutoring. (Full disclosure: Veritas and Integrated Learning are partners of ours.) You may not
need a prep class -- a good book or two is all that a lot of people use -- but I'd recommend checking those out if you consider going that route.
Scott