If you want to base it on popularity then you should use Debian. Debian and its child distros (of which Ubuntu is one) make up the majority of Linux distros and the child distros are still 99% Debian.

Flatpak is available on every distro.

Ehhhh

Professionally I've only ran into a handful of Ubuntu installs.

Dozens of SUSE

Hundreds of thousands of RHEL.

So if I wanted to help someone new, I wouldn't recommend Ubuntu because it would be somewhat of a dead end.

Fedora gives you familiarity with the largest deployed commercial Linux, while still getting the newest packages out there through either fedora yum or flatpak. Best of both worlds.