One of the strongest One Piece characters, Whitebeard, risked his life in order to save Ace from execution.
Whitebeard was even more powerful in his prime. He belonged to the Rocks Pirates, which disbanded during the God Valley Incident.
Shanks meets Whitebeard in chapter 434 and asks him to stop Ace from going after Blackbeard. Because Blackbeard's abilities were well known to Shanks. Whitebeard first talks about his scar at the meeting.
When speaking with Shanks in chapter 434, Whitebeard mentions that the mere look of him makes his scar hurt. Of fact, it wasn't Shanks who did it; it was someone who looks like him.
Also, there is a man connected to the God Valley incident who even the world's worst criminal, Monkey D. Dragon, feared. The leader of the God's Knights, Saint Figarland Garling.
When he executed a Celestial Dragon in chapter 1086, he made his first appearance. Even though he is old now, chapter 1095 includes a flashback of the God Valley Incident, which happened 38 years ago.
The similarities between Garling and Shanks are clear in the flashback. Although the show has never stated it, it wouldn't be shocking if they are father and son because they look absolutely alike.
Garling is the only character in One Piece who fits the requirements for Whitebeard's scar. His power cannot be ignored, the fact that he was capable of seriously wounding Whitebeard in his prime.