My assumption was WB talked about the wound in his HEART... where once he sees Shanks scar in his face... he remembers Blackbeard who gave him a hurtful scar by killing Thatch.... that's what I've always had in mind.... and not the scar on his chest...
As for the physical scar on WB chest, I will nominate two candidates:
1- Big Mom (because Marco said: I know first hand, how dangerous you can be... so maybe they clashed in past, and Marco witnessed getting a severe injury by her... I think WB won eventually, but still, this might be what Marco eluded to)
2- Crocodile, maybe this was the achievement that made Crocodile be offered Warlord status as mentioned by a member here in forum... Also, WB skin is not as tough as Kaido/Big Mom... so maybe Croc can achieve that by normal means.
I wanted to put Roger or Xebec, BUT Roger died before WB got his scar, as for Xebec, although I still believe he is alive, I DO NOT believe he made a move after his disappearance, I think he stayed behind the radar without doing any action, not even meeting his right hand man, WB... Although, this could be a case made, where he wanted to make a comeback, and went to see WB, invited him, got rejected, faught, and WB got that scar, but Xebec couldn't subdue WB a second time... But I doubt Xebec made a move to begin with YET...