I have been to quite a few National Parks - Big Bend is BEAUTIFUL, but the caveat is "IF you like Desert landscape". If you aren't into Desert (South-western US) then it isn't very attractive as a NP. The hikes are great, it is very hot there, not much water around so you have to pack your water everywhere, but it was a very fun trip overall.
Yellowstone and Yosemite for me are my top 2.