Must try distros: Fedora, Mint, Void. But seriously, if you are using Nix to begin with, why use anything else? Nix is as good as it gets. If you really want to do a combo, I would recommend Fedora or Mint using Nix as just the package manager and not the hypervisor. All distros are basically the same nowadays.
Must try desktop environments: Xfce, Cinnamon, Gnome, KDE Plasma