there are 2 that are easily the most brutal.
A proper neck crank. Black eyes and broken jaws suck, but damaged nerves in the neck are for life.
By far the worst are the kind of slams Quinton used to do. He's lucky he never killed anyone, the g-force directly to the brain is the highest physically possible during a slam on the back of your head. On a sport science show they proved Quinton's slam create the greatest impulse in any move in any contact sport.