Death Valley, California. Definitively one of the most impressive places I have seen so far in the US. As a matter of fact in the world. Some facts about the Death Valley:
The lowest point in North America (-282ft)
One of the hottest place on earth, the hottest temperature ever recorded in the world was 134 °F on July 10, 1913 in Furnace Creek, in the bottom of the death valley.
The biggest National Park in the continental US.

This really is the desert to its extreme. Very impressive rock formations, especially in Zabriskie point, a few sand dunes (not a very common thing in the American deserts), and lots of large open spaces. Of course the vegetation is very limited there, it is also one of the driest places on earth after all.

You can drive on this kind of roads (left side of the bottom picture) for hours without seeing any other soul. Speed limit 65mph, quite high for a road which is not a freeway. One curve just about every 15 minutes and the distance between two gas stations can be over 100miles. I have to get used again to those rotten L.A. freeways now.

I posted my pictures of the death valley at this link