If you prefer to understand how your OS work, and want a system simple enough that you can understand what it's doing, go with Arch. If you don't care about that and just want something that mostly works with little hassle, Debian, Ubuntu, Suse, or Fedora are all fine; there's little practical difference, except maybe Debian's software is always a little out of date.

I'd say only go with NixOS if you want to learn the Nix language, specifically need reproducible builds, or really hate mutable state.