I can't pick one.
For me it's like asking if I like oranges or apples better. I like them both.
The desserts if AZ were beautiful, but so are the forrested mountains of PA, or the farmland in PA seen from a small plane, etc.
I've seen Niagara (I like the Canadian side best) but wouldn't call it the MOST beautiful.
They're all beautiful in their own way.
Sorry if that kind of dodges the question, but it's true.

I know you asked for a place but I've seen so many striking and memorable places to chose just one while an honest and spontaneous smile on the face of someone I care for is beyond compare.

All the canyons out west.

IN 1978 I was doing some geological research in Nayarit, Mexico. We're coring volcanic lakes for sediment samples. Our next visit was a volcano known as, Lago de Santa Maria. We approached the crater from the west. As we broached the rim of the volcano we looked down. Below us about 0 ft was a turquoise lake. The surrounding rim from the top to the lakes edge a 0 ft below was almost unbroken jungle. Only one habitation, long abandoned and the road we were to descend was intermittently visible. We followed the road to the lakes edge. The water was crystal clear, with fine volcanic sand extending as far as the eye could see into the depths of the lake. There was absolutely no plant or animal life in the lake. Minerals from the volcano rendered the inside if the lake devoid of life, in stark contrast to the lush semi-tropical forest surrounding the lake. The water was about 70 °F. Needless there was no need to sample the lake sediments. So we put our plywood drilling platform together, pushed it out on to the lake and spent the day swimming, sunning and watching the Anhingas spread their wings and sun themselves slong with us around the shore line.

The Redwood forest where it meets the rocky shore, at sunset

I'm partial to Yosemite Valley, but there's beauty all over.