Realistically, it’s anyplace where I’m away from the rat race and enjoying beautiful views.
That being said, some of my favorites have been a KOA near Glenwood Springs, Colorado where I got a spot right on the Colorado river. It was really pretty and the sounds of the river and deer walking through the campground made it a fun trip. In addition, honorable mentions go to Lake Cachuma near Santa Barbara, CA and Crater Lake RV resort in Oregon. Both were near water, had pretty views and they had tons of activities.
Maybe not the best, but the coolest one I've stayed in is City of Rocks State Park in New Mexico. The campsites are nestled in among these giant rock formations. Very cool place.