I'm happy to be proven wrong, but I though Paranal beat Mauna Kea (and some basic google searches aren't throwing up anything that makes me question it, e.g. https://www.eso.org/gen-fac/pubs/astclim/espas/espas_reports..., though that's from more than 20 years ago, so site quality has likely changed since then). There's also the issue of northern vs. southern sky.