Gentoo. The way you can change build options for individual packages and have it reflected in the resulting dependency graph is pretty unique. Most other package managers essentially have static dependency graphs and sometimes offer you separate packages of the same program as a workaround (like Emacs and Emacs without GUI support).

I've tried NixOS as well but I didn't stick with it. It does so many things in a unique way that there's no way to use it without constantly cross-referencing the Nix documentation with the upstream documentation and digging into the Nix code to see what on earth they're doing. The error messages are completely useless and it needs a lot more documentation. It is very cool though.