Edit: wow, this is a never ending comment section!
NixOS
I just heard of NixOS for the first time because of this thread. Looked up some videos on it, and my jaw hit the fucking floor.
I really liked https://www.youtube.com/watch?v=AGVXJ-TIv3Y - step by step, with examples and great explanations. Warning: it’s long, but I watched it in one session.
Gentoo because it can do it all
OpenMediaVault
Good OOTB customizations, works on Pi, and easy to extend with plugins (Docker/Portainer is pretty much all I needed).
Good ol’ Debian 12
Ubuntu server, I want to switch to debian but I don’t know if it’ll be worth it