The canyons in New Mexico at dusk are awsome.

On a platform on the outer Great Barrier reef watching the sun go down in the direction of the mainland after the last boat has left and you get to stay the night.
[quicksilver-cruises.com]

So hard to to say, but the most recent most romantic spot would have been in Thailand , the "Our Jungle Camp", a tourist stop in the Khao Sok National Park, we stayed in the last hut of a dozen at the end of a walkway, on high stilts so you are in among the trees; were you enter the camp there is this lovely restaurant/ bar/ hangout house you can play games in, or sit and read, or drink, all staff super nice and chilled, a river winding its way round just between you and a massive vertical limestone cliff, going what feels like half a mile up into the night sky. Lots of animal noises to be heard at night. Nothing else.. Full moon illuminating that cliff face and the wet trees, as rain had just fallen. Everything perfect.

Had to be the overnight trip on a local fishing boat taking us to Komodo Island from Flores in Indonesia. We anchored off an uninhabited islet during a beautiful sunset. During the afterglow, fruit bats flew from the islet to Flores, so many of them they almost blocked out the light. Afterwards, the fisherman fixed us dinner (a fresh local catch, of course), and we all shared a bottle of wine.

Trevi Fountain in Rome at night.

Laying in the desert of Utah watching meteor showers