I vote for Yellowstone...
Of course, I haven't been to many National parks, but last year's trip to Yellowstone was absolutely awesome! We took the Beartooth Scenic Byway from Billings, Montana to the North East entrance, which was a beautiful experience in itself, taking you to the 10,000+ ft. high peak of a mountain. The geysers, hiking trails and wildlife of Yellowstone were just a wonderful experience. I'd recommend staying near Old Faithful (two suggestions: Old Faithful Snow Lodge or Old Faithful Inn) because you are so close to many of the geyser basins. Don't forget about the waterfalls on the East side of the park in the Canyon area; I think many people overlook that area. Also, we went in September just after Labor Day...perfect time to go. Extremely low crowds and the weather was great.
If anyone needs any info about Yellowstone, feel free to PM me and I will try to help!
-Laura