If the distribution does not have it by default, please include the instructions to use it on the system.
Note: I can’t compile the libre kernel from the source.
Short answer is Trisquel if you like Ubuntu/Debian, Parabola if you like Arch, and Guix if you like frustration.
The libre kernel is a bit of a pain regarding wifi and bluetooth, and depending on your graphics card the drivers aren’t going to run quite as well. You might need to get new a wireless card/usb, since there’s only a few modern chips that work with it.
There’s a list of distros on gnu.org that use the libre kernel by default, if you want to look at more options. PureOS is based on Debian focused on privacy and security. Hyperbola is based on Arch with 32 bit and BSD options.
Personally I use Guix, which is an amazing abomination with awesome features that most people don’t care about. I wouldn’t recommend it for most people unless you are coming from NixOS, know a lisp dialect, and/or are willing to put in a lot of effort.
Trisquel provides a good experience out of the box imo, as long as your hardware is supported and if you don’t mind the dated looking interface. I used it for a while on my corebooted laptop.
I didn’t used much any other “100% libre” distros. As much as I wanted to use it, I never managed to have Guix to run on that machine.
[edit:] to answer OP’s question, I would use a distro that ships with it.
