If been to quite a few observatories, including those on Pico del Teide at an elevation of 2300 m at Tenerife, an island near La Palma.
In winter, the best skies I'v seen were in the German High Eiffel, at a mere 700m altitude.
Also, above 1700m, I've seen some amazing skies both during summer and winter in the Austrian Alps. Both sights presenting mindblowing views in even my modest 10x56's, easily rivaling the views at Pico del Teide at 2300m. The difference is the generally much better and more consistent good weather and clear skies at the Teide. But to the Eiffel, I even brought my Questar 7, making the quality of skies + quality of instrument equate to a much better result than my small ST80 at 3000m next to some major observatories did.
If you can safely travel far, Mauna Kea or the Mountains at Cerro Tolole in Southern Chili will best them all.