If you have never been to the Yellowstone area, that is where I would go. Yellowstone park is beautiful and it is awesome to see all of the geothermal features and to view the wildlife. There are many great rivers to wade within the park and many more outside the park. There are five or six fly shops in the town of West Yellowstone and the all offer guide services.
I have hired a guide to drift down the Madison River (outside the park) several different times over the years. It is a great experience. The rainbows and browns are plump.
Everytime I have wade fished within the park, I have had close encounters with Elk, Bison and even a black bear once. It is amazing. The Yellowstone area is my favorite place to fish in the lower 48. Now that I have been to Alaska, nothing can beat that!!!!