Which agent do you think is the best and the worst shooter lore wise?

I would say that Sage is the worst shooter. She doesn't seem to be very trained with gun and more focuses on helping others with heals and stuff. Yes she is determined to fight but other agents seem to fit more into the shooter category.

And I think Chamber is the best shooter. He literally invents guns and uses them in the battlefield with full confidence (see how he holds headhunter with one hand - terrifying to go against I would say). Brimstone was also probably a great shooter, but he's well past his prime.