>What is the best action
Depends on what you want.
Do you want to build an MG that is supposed to be able to shoot a couple hundred rounds in a few minutes without malfunction?
Recoil operation. It's so fucking simple that only broken ammo can stop it, and there is nothing that can survive bad ammo.
Do you want a lightweight system that functions even with extremely low power loads?
You probably want manual bolt action then. It frees you to experiment with different bullet weights and loads, and allows you to experiment around.
It all depends on a lot of different factors, and while there are systems that function similarly and achieve similar results, all of them were designed with unique ideas in mind.
Obviously that doesn't mean that there are no bad obsolete systems.