I have not been to all of the National Parks in the US but of the ones I have been able to travel to I have found Glacier National Park in Montana is the most beautiful one I have seen so far.
Apakuni Falls
Apakuni Falls trailhead
Aspens
Avalanche Lake trail
Going to the Sun Road
Logan Pass overlooking Going to the Sun Road
St. Mary's Lake via Sun point trail
Lake MacDonald