For NP that I've been to (I've been to almost all of them in the Western US) it's a toss up between Yellowstone, Glacier, and Rocky Mountain, Yosemite is great but WAAY to crowded, even in the backcountry. Though all of the southern Utah parks are a photographers dream too. (Except for Capital Reef, it's the only NP I've been to were I though "this is a bit lame"). Grand Canyon doesn't excite me much, just a big dry, hot hole in the earth. But I am planning a trip to Havasuipi this spring, it's connected to the Grand Canyon but is on reservation land.
Most of my backcountry time is spent in the Wind River range in Wyoming however. I work every summer there as a backpacking/rock climbing guide for youth groups. The Wind Rivers are great, it's like the better known Teton (snicker, snicker those dirty French) range, only higher, 8x the area and less crowded. Second for time spent for me is in Yellowstone, I've probably hiked 75% of all the trails in Yellowstone, Bechler Canyon being far and away my favorite, it's a waterfall lovers dream. Stuff like this every few miles:
Although Mammoth in the wintertime is awesome to, -25°F and steaming hotpots make for some neat things like this
