Who is the GOAT of Rugby?

Hi! I’ve been learning and watching rugby since 6 Nations tournament started, and one thing I want to know is this. Just as how Messi and Ronaldo are hailed as the GOATs of Football, Novak Djokovic the GOAT of Tennis, who is considered the GOAT of Rugby, and why? What’s the criteria? Is it penalty kicks, tries, points, or all of them? Curious to know…