Roger's first and perhaps his most formidable rival and greatest enemy was Rocks D. Xebec, the captain of the legendary Rocks Pirates who sought to become the king of the world.
Garp, said to have cornered Roger multiple times before his surrender, as well as Roger's second foremost pirate rival Whitebeard, who held the power of the strongest Paramecia Devil Fruit and was called the "Strongest Man in the World", even before Roger's death. Another person known to rival Roger was Rocks D. Xebec.
Xebec, commonly known as Rocks, is a posthumous antagonist of One Piece. He was the captain of the legendary and notorious pirate crew called Rocks Pirates. He was the arch-nemesis of the Pirate King, Gol D. Roger.
Kozuki Oden
He was the second division commander of the Whitebeard Pirates.
However, Sengoku recently stated in One Piece Chapter 957 that Roger's most formidable enemy was the Captain of the Rocks Pirates, Rocks D. Xebec.
Roger's first and perhaps his most formidable rival and greatest enemy was Rocks D. Xebec, the captain of the legendary Rocks Pirates who sought to become the king of the world.
Roger and his men, as seen during the Oden flashback, would simply be too strong for Luffy to defeat in a fight.
1 Whitebeard, The World's Strongest Man
Captain of the Whitebeard Pirates, Whitebeard was one of the strongest pirates of Roger's time. With the power of the Gura Gura no Mi and an incredibly strong Haki, Whitebeard was known as the strongest man alive. The only person strong enough to equal him in combat was Gol D.
Portgas D. Ace, born as Gol D. Ace and nicknamed "Fire Fist" Ace, was the sworn older brother of Luffy and Sabo, and the biological son of the late Pirate King, Gol D. Roger, and Portgas D. Rouge. Ace was adopted by Monkey D. Garp, as had been requested by Roger before his execution.
Locations: Loguetown. God Valley. Skypiea (Upper Yard)
It is unclear whether Rocks possessed some kind of devil fruit, but it would be plausible to assume that he did. Many theorists have predicted that Rocks possibly had the Goro Goro no Mi, a logia type devil fruit that allows its user to create and manipulate lightning.
Rock D Xebec was killed by Imu : r/OnePiece.
It's none other Rocks D. Tar. The World Government is aware of this fact and tried to cover up his real name, just like they did with 'Gold' Roger, and left out the D. making his new name just; RocksTar.
He is also part of the 11 troublemaking pirate rookies called the "Worst Generation", alongside with his archenemy, Marshall D. Teach (Blackbeard).
Gol D. Roger was executed by the world government. Execution is a pretty common punishment for piracy in the One Piece world. (That, and a prison sentence to Impel Down for life without parole.)
Conquerors Sickness. So we all know Gol. D. Roger died of a sickness.
The true reason why Garp couldn't save Ace is his affiliation with the Marines, which is also his defining characteristic.
In the Post-War arc, one of the Five Elders stated that "D." means danger. After the timeskip, Law mentioned the Will of D. once more when Doflamingo asked him why he had so much faith in Luffy.
When Ace was born 20 years ago, Roger was 4 years dead. All the inner turmoil and resentment he carried because of the treatment he received, from being the son of Roger, was all for nothing.
Whitebeard, on the other hand, easily equaled Roger's power. Even Garp called Whitebeard "The King of Seas." So, it is beyond any shadow of a doubt that Whitebeard could have defeated Garp in a fight to the death.
Luffy got his scar on his chest from Roronoa Zoro this happened while Zoro's being controlled on Episode 223 from 10:55 - 11:10.