Linux

If we allow derivatives, I'd say SteamOS despite being Arch. It's putting Linux in non-technical people's literal hands and it's not a locked down and completely different platform that happens to run Linux like Android is. It's almost designed by Valve to give people a taste of Linux by the addition of its desktop mode, and people that would be modding consoles are now modding SteamOS and learning how much fun an open platform can be. I've seen people from sales talk about their Decks on my work Slack.

Otherwise, NixOS, no contest. It's been a really long time since we've last seen a fundamentally different distro that's got some real potential. For the most part, Arch, Debian and Fedora do similar things with varying degrees of automation and preconfiguring your packages, but they're still very package oriented. We've been mostly slapping tools like Ansible to really configure them to our liking reproducibly, answer files if your package manager has something like that. And then NixOS is like, what if the entire system was derived from evaluating a function, and and the same input will always result in the exact same system? It's incredibly powerful especially when maintaining machines at scale. Updates are guaranteed to result in the exact same configuration, and they're atomic too, no halfway updated system the user unplugged the system in the middle of.