Central Divinity Wong Chung Yeung proved himself the greatest of the first generation of Greats at the First Mt. Hua Sword Tournament, and by the end of ROCH, Gwok Jing was conditionally the world's best martial artist (disputed, but not to the point that it is an impossible or even improbable claim).
So if Central Divinity Wong Chung Yeung were to fight Gwok Jing at the latter's peak late in ROCH, which one would emerge as the "Greatest of Greats?"