I think it would be John. I doubt Ricketts was half as good as makes himself out to be (which is implied with rumours of him shooting people in the back), and the other gunslingers associated with Calloway are either Black Belle - who seems pretty good but not as good as the player - or past their prime and difficult to evaluate.

It's got to be John or Arthur depending on how the player plays each character in the two games. If you take care of most RDR2 side quests as Arthur, his initial skill advantage and Guarma experience might put him over the top. John might edge him out regardless because of RDR1 (which I admit I haven't played all the way through), but especially if you do a lot of the gunslinging side-missions as John.

Either of them is far more interesting than any gunslinger in the real world. People became famous in the West based on surviving two or three gun battles or killing 5-20 opponents. Arthur and John have survived dozens of battles and killed hundreds or even thousands of opponents. Arthur was stranded in the Caribbean for a week and managed to turn the tide of a revolution and personally kill a dictator!