What in your opinion was the most powerful of these alpha predators?
I believe it was T-rex, not by size, but by its strength and weaknesses. Tyrannosaurus had superb senses, meaning if it was blind it could still smell out prey, it had the most powerful bite force of any known land animal, it could crush bone, it was reasonably fast, 15-25 miles per hour, and was tough enough to battle most herbivores of its time.
These are my reasons, what are yours?