NixOS. The declarative principle is very nice (especially the installation process) but as soon as you do something advanced like compiling stuff or using software that is not in the repositories or outdated, it’s not very simple to use as you basically have to become your own packager and maintainer. Recently I tried upgrading the main PC of my hackerspace from one NixOS version to the next and we had lots of breakage, because the migration paths are not automatic and you have to change your configuration even though upstream didn’t change anything. Upgrading my Debian home server from 11→12 was as simple as changing the sources.list, running apt full-upgrade and answering a few questions.