Fedora Silverblue. But since Fedora aims to include telemetry (although in a reasonable way) by version 40, I'll switch soon to something else. I feel it might be time to give BSD an honest attempt.
There already are a few: https://itsfoss.com/immutable-linux-distros/
and Ubuntu seems to work towards an immutable future as well, I think it's where most distros well be in a few years. The transition needs time though.