As someone as already mentioned, for certain Larry Holmes in his prime. He possibly took the flushest single punch from possibly the hardest hitting Heavyweight of all time, and he got up in like 4 seconds.

Muhammad Ali also took an amazing lunging left hook from Joe Frazier, and was up straight away.

I think those two are really a given, but other notable ones are David Tua, George Chuvalo, Oscar Bonevena, Evander Holyfield, Oliver McCall, Ray Mercer, ETC.