The main problem with visibility is that it’s REALLY hard to forecast properly.
Absolute majority of numerical weather models do not have enough resolution to forecast fogs for example.
But when we’re talking about visibilities 20 km and greater, I believe Meteoblue would do a pretty good job at that.
I work at an airport and there are definitely days when the visibility is around 20 and sometimes even close to 100, mainly with an advection of a crystal-clear oceanic airmass after a cold front.