Yellowstone National Park has a high frequency of bison.
Yellowstone also has wolves but they are SUPER rare, but we were lucky enough to see one in the fields, maybe it was two but we weren't sure.
Yellowstone National Park also has geysers, hot springs, and it has biscuit basins - which are a group of geysers together.
I suggest going there in autumn because its not crowded at all because in the spring or summer it's packed with people and you can barely get around and see things. I think I would go there again, if you go there in autumn make sure to pack warmly. : ) I love the faces :P