There are a few for me but the first view of the Grand Canyon blew me away. Even though I lived in the west most if my life, I’d never been to the GC until maybe 8 yrs ago and was blown away by the size. I've been a few times since and it's not the same as that first view where it was just hard to comprehend the size.
I’d put NYC/Manhattan up there too. I didn’t understand the hype until I went there. I would not want to live there, but it is a great place to visit.