Red Harlow is the best gunslinger in the series. He would easily kill Arthur, John and even the legendary Landon Ricketts, here's why:
Even though the game is almost 12 years older than RDR2, Red still very much comparable to Red Dead Redemption 2's engine. The duel system is solid, and Red is probably a faster shooter than Arthur, even though Red Dead Revolver's engine is 12 years older than RDR2's engine. So if you compare them gameplay wise, Red still holds up to both John and Arthur.
And if we compare them based on their feats, Red will destroy all three of 'em. Let's see:
Red took out multiple gangs, he killed a 4x duel champion in a duel, he pretty much took out the Mexican army and killed hundreds of US Soldiers. He killed dozens of US Soldiers and the governor in the last mission, he hunts down people like Arthur and John multiple times in the game. And on top of all of this, he did almost everything single handedly. He had the occasional help from people like Jack Swift, but he's mainly on his lonesome. And the ones who help him are completely useless anyway. And he's also the only protagonist who doesn't die.
There are very few people who've played Red Dead Revolver, but most people who've played the game agrees that Red is a better gunslinger than Arthur and John. This is obviously not gonna change anything in the "Who's the best gunslinger?" polls. And I am aware of Red Harlow not physically existing in Red Dead Redemption, but he does exist in some way. He's a myth in Red Dead Redemption.
MUST READ: Uncle is NOT Red Harlow! I don't even know who or why someone came up with this, Uncle's personality is the exact opposite of Red Harlow's personality. I don't get why anyone can even suspect that Uncle is Red Harlow.