This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
"Best" is totally subjective. You'll need to list some real criteria if you want anything more substantial than just people telling which one they use for what may be completely different reasons than you might have.
Different libraries have something different to offer new/different to offer although all of then end up parsing/creating the JSON. I remember this query being asked in Java In General forum, please SearchFirst for the same.