Baer being the hardest puncher of all time, is a matter of opinion and probably not a concensus choice at that. Louis was certainly a great combination puncher and overall technician, but he was barely 200 Lbs and managed to dust Schmeling in only a single round. Additionally, Schmeling had other losses that came against fighters whom I'm not so sure were terrific, and the physical profiles of his victoms have about as much in common with Vitali Klitscko as water does to oil.
I have seen Audley Harison take some real bombs and shrug them off. The bottom line is that a chin has to be proven over a large number of fights at the top level.
It is probably not relevant, but if you posed this question to Vitally he would almost certainly pick Schmeling. VK idolises Schmeling and even named his son after him.
People say stuff like that all the time mate. It's only to avoid looking bad by speaking ill of a predecessor. If I were middleweight champion of the world today, I wouldn't be runnin' around saying " I'd kill Sugar Ray Robinson, what a ***** ", It just wouldn't be proper.