Louis probably had the best shoulder snap. Or maybe Dempsey but we don't have good enough film of him to know for sure.
By the way, this is a shoulder snapping straight right. And a very short one. Imagine his hooks and overhands. Woo lad
Golovkin generates power in a way that most boxers don't know about. Most boxers believe that hand speed, hips, and pushing off the ground gets you power. But there's something else you need to have elite power. It's called the shoulder snap, and it's when you produce and explosion like sensation in your shoulder when punching. It's a technique. Once you know it, it requires very little energy. And it's a whole sub-craft of punching. You can apply the shoulder punch to any punch, the jab being the hardest. I'm not sure if it's healthy though, as it shakes your whole body when you punch.