Considering that Katakuri said its been years since he had been punched, I think it was probably more likely to be Smoothie or Cracker...
well most likely it is Snack there are reasons for that:
first of all Cracker was surprised that Luffy was able to destroy his armor , and Smoothie should be stronger and without a doubt Katakuri is not the one, otherwise he could have mentioned that in his fight with Luffy , he is the strongest sweet commander .
secondly Snack as he was introduced in chapter 894 he was angry and talking to himself mentioning the worst generation , i doubt it is only because of Urouge , i think BM degraded him because of his apparent weakness against the supernova