I've done the Badlands and Greyrock.. The badlands is awesome due to it's central location of parking, camping and trail riding. You can get from most any trail back to the parking lot or camp site in just a matter of minutes. A great mix of climbs, rocks, sand water and mud trails gives you a great mix to test your rigs ability...
Greyrock also has a cantrally located camping area with trails branching off in all directions. In my opinion, more difficult trails than the Badlands, with huge hill climbs of loose dirt and rock that will unnerve all but the best.
Zig