The first obvious answer may be the one you are comfortable with but that may be none for a new computer/ linux user.

The one where those package are available. Or even multiple ones.

With atomic (immutable) distros this question will arise more often as the question of which image becomes more important than which distro to chose.

I use dnf but I could also use apt or pacman. What are the relevant advantages of the package managers? What’s the reason to use apt over dnf? Nix? Simply package availability?