NixOS so I can keep my config in git. I have a single nix config for all my machines (desktop, laptop and server) so I can share configuration between them. I use it to configure both my system and my user config, my dotfiles, with home-manager. Even my neovim config is in nix thanks to nixvim.

I don't think I could go back now. It can be a bit of a pain from time to time and the learning curve is steep but it has so many advantages. Being able to rollback between config versions (called generations), having a consistent config between my machines, having it all in version control… The repo have so many packages and when there is a module it's really easy to add a service. Writing new packages (derivations) and modules is also not that hard. It can be as simple as calling nix-init.

Had my main ssd fail on me a few month back and it was very simple to just replay the config and just get everything working as before. I only had to do the partitioning by hand (it can be done by nix but I've not gotten around to it yet). That's why I only backup data and home partitions, not system partitions.