As a Cov Cath fan, I am biased, but the greatest comeback I ever saw was the 1997 Mud Bowl in Ft. Thomas. Highlands had completely dominated the first half. before going to their locker room at halftime. CCH went to the North endzone in the pouring rain.
Highlands returned to the field in fresh, clean, dry uniforms to start the 3rd quarter and proceeded to try to score 100 points. In the process, they allowed the Colonels from Covington Catholic to steal victory.
Greatest comeback. That's the greatest I ever saw.