I'm going with Utah and Wyoming coming in a close second. Come to think of it though Nevada has a lot of open desert, as pointed out. California doesn't have the most but there's quite a bit. Sure is good to live in the west!
If I had only a visit and one chance to do a trip like that, hands down I'd go to Moab Utah. The scenery is unparalleled and the riding is as easy or as hairy scary as you want it to be.
I not sure how you could truely varify how much each state has. I live here in Nevada and there are thousands of miles of roads,2 track and trails to ride. Utah and Arizona are close im sure.