Re: Has there ever been a puncher...
Marciano said something to the effect that if he faced Tiger Ted Lowry a hundred times, he'd have to go the distance a hundred times.
No, there has not ever been an irresistible force in boxing, just immovable objects.
As for Tex Cobb, it might be worth keeping in mind that he took all the hardest kicks from "Big John" Jackson in a PKA match without wobbling, and Jackson was known for his ability to take out world class opponents with his kicks. Nobody has a punch which can measure up to a hard kick from a skilled HW martial artist, not even Earnie Shavers.
Vitali Klitschko also has a background in kickboxing. There was never any possibility that Lennox Lewis could ever hope to hit him with a punch as hard as kicks he had already withstood numerous times.
Assuming one is equally skilled at striking with both fists and feet, fists might be more convenient to use, but I would suggest that it's considerably easier to bash a huge dent in a metal door with a spinning back kick than a fist. (How many boxers have you seen flatten a world class competitor with a punch square to the sternum? It's somewhat more common to do this with a good hard kick to the chest.)
There have been martial artists who were impervious to the hardest kicks, and it goes to follow that they would likely be impervious to weaker punches as well.