I went to Blackberry Mountain a few days ago - the setting is not as spectacular as some of the other hotels mentioned here and the “nature” experiences are pretty normal (eg hiking, fly fishing, horseback riding...etc) but the hotel can definitely be considered luxury even by the standards of this forum. In fact it’s probably one of the most well-run resorts I have experienced in the US.

I also really like The Point in upstate NY (even better now as they got rid of the communal dining experience due to covid).

Outside of the US, I enjoyed both Explora resorts in the Sacred Valley and Atacama Desert.