I'm giving him an S for several reasons.
First, he had probably one of the best chases for a world title in years.
Second, he actually has a great character, not as over the top, but a likable character with clear faults and a relatable story of gaining confidence and belief in oneself.
Third, He has improve greatly over his time in AEW, and even in the years beforehand.
Finally, the quality of his last 4 matches pushes him over the edge into an S tier.