I think the nicest one imo was Yellowstone National Park. Went there in 2010 with my parents. Had no idea initially it was that huge. My only disappointment was not seeing a bear lol. But saw plenty of other wildlife. Just beautiful.
I've got a soft spot in my heart for these two being their sort of local to me:
Branch Brook Park in Newark, NJ. Peaceful and underrated place in general. Nice place on the outskirts of an urban city. Beautiful basilica Cathedral of the Sacred Heart is right there as well. I did the whole set up. Plus cherry blossoms come out there around March or April. That's quite a sight.
Fort Wadsworth Recreation Area in Staten Island. Another underrated and peaceful place a lot of people dont know about. Beautiful view of the Verrazano Bridge. An old Fort during I believe the American Revolution is there as well for private tours