The foremost place for observatories is on the Big Island of Hawaii. The Moana Kea Observatories are at the summit, over 13,000 ft. There are 13 telescopes funded by 11 different countries.
The reasons are simple, very dark skies from the lack of light pollution, high elevation and the low humidity at this area.
It is easy to get to Hawaii and also to get a tour at the summit, sorry no chance to view at an observatory unless you have made very special arrangements.
Very cold up there, with snow year around, our tour provided heavy parkas.
I was there last year, we stopped for an observation viewing at the visitor center down at mid mountain.
The star show is incredible.