I just recently started their online class, which comes with all the books, and I've been through 4 of them so far. Honestly, I think they're all really good. I'm so out of practice with almost everything on the GMAT that these books have been priceless for me. I guess you have to decide what you need the most work in, but it's defintely math for me. They cover everything from the basics to the difficult AND show you how to break everything down into more simple questions. If I had to pick which ones I would get, I would say go for the math-related books. Also, they assume you have all 3 OG books.
I know you can find them cheaper on Amazon.com and Half.com. Just go to the Manhattan website for the exact titles and you can find the books elsewhere.