Ubuntu 100%, if you count how many distros are ubuntu based (and collaterally debian based), but I believe it is the most used one even if you only count official ubuntu releases
Maybe arch would be quite high, if you count the steamdeck as desktop (maybe), and the big increase on arch users in the past couple of years (wen't from being rare to 1 in 3 users saying "I use arch btw")
It's honestly sad, almost to the point where defederating from them would be better
But I'm sure we'd be loosing good people from hexbear too