A Tri-Ball with a recovery hook is the best for this.
It's a very wide area that means it will stop the other car better before it hits your bumper. If they go far enough that it goes through the radiator, the recovery hook will rip out the guts of their cooling system and front grill on the way out.
... I've been rear ended too many times with one on my old work truck, where I hauled trailers daily. I installed one on my personal truck as protection. Haven't been hit yet on it, but in case it happens.