The default one is different per operating system or desktop environment. On KDE it's Breeze, on Windows it's Colibre, on macOS it's Sifr (off the top of my head).
I just checked, the one I was using, the default on NixOS with Sway, was Elementary, which I didn't like. I will admit that I liked the other built-in ones better, but I still like OpenOffice's better than any of them.