Impossible to pick one but several US national parks such as Yellowstone, Yosemite, Grand Canyon…so breathtaking and unique. Many locations in Hawaii from coral reefs to volcanos.
My first thought was Grand Prismatic, just after dawn.
Cool enough that the thermal features create a fog over the water, and more importantly, all the tourists (I was a hotel employee in the park) were still in bed. Had the boardwalk area all to myself.
I think I just sat in contemplation for half an hour, which if you knew me would say is impossible. Very peaceful.