Linux.
But of course I need a desktop UI too so that alone isn't enough. I don't have a favorite though.
Windows has a decent core and good core UI, but makes it awful with win11 UI and product pushing. I'm being pragmatic, not enthusiastic, using it.
Ubuntu has or had PPA for selective more direct and up to date software, but I guess with the newer package distribution formats (flatpak and the others) I guess that's not necessary or a comparative upside anymore.
The UIs I tried or used on Linux I never really liked. It was reasonable or acceptable at most. I wonder if there's one I'd like out there.