Probably John Marston. His accomplishments with a gun are more than what the likes of Landon Ricketts ever managed, and (well, in mission canon, at least) Marston doesn’t shoot anyone in the back.

Based on RDR2, every shooter is exaggerated at some point, but objectively at the core of the game there is no one better than a borderline shooter, because everyone has succeeded in something. The same Arthur was able to kill several legendary shooters who were actually much more experienced than him, could also simultaneously kill opponents in packs, and John, also killing a mountain of opponents like Arthur, was able to surpass the Ricketts in skill. I think this question will remain open.

I bet on Jim because in the game he is the most difficult opponent in a duel.

I certainly agree with Arthur and John being more skilful than the gunslingers depicted on the cards. Think about it, if, say, Landon Ricketts achieved greater things and had better exploits, the game would be around him; it’s not, and instead, he’s rumoured to have shot people in the back.

I also think that Micah is up there too, just behind the level of Arthur and John. The massacre in Strawberry, the gunfight in Rhodes, his reign of terror in 1907... Micah is guilty of many things, but being an incompetent coward is not one of them. He is demonstrably a very skilled shootist, insofar that even John had no guarantee of beating him had it not been for Sadie.

As for Calloway, while he may be the fastest duelist in the game, it’s important to remember that he does lose. Yes, he may have been (and probably was) faster once, he’s not as quick as Arthur or John. Given that various sources say his exploits are exaggerated, it’s a fair guess that he was never as good as Arthur, John, Micah, etc, even when he was in his prime.

I also think John is the greatest not just gunslinger but all round the greatest character but for gunslinger Landon rickets has to be second he’s the fastest and red Harlow needs to be mentioned

^Red Harlow doesn't exist as a real person in the Redemption universe.

Yup.