What is the most accurate caliber you have ever shot?.
I don't think you can answer that question.
I think its the shooter more then the caliber. We can shoot some calibers better then we can others, IN DIFFERENT ENVIRONMENTS, and some platforms are easier to shoot.
For example, lets take the Service Rifle. People found that they could shoot the M14 better then the M1. But the '06 can be made to shoot as well as the 308. People even shoot the M14 better then the 308 chambered M1s.
The comes the ARs, They are out shooting the M14s in service rifle. Even at 1000 yards. I don't think the 223 is more accurate then the 308, but it's easier to shoot. People are getting distinguished earlier, and at younger ages now days, more woman and youngsters are getting their badges, not because of the round is more accurate, but it's easier to shoot.
I use to shoot a lot of 1000 yard matches where we shoot 4 matches in a day; Any rifle/any sight, any rifle/iron sights, Service rifle and team match.
I used a Model 70 300 WM in all but the service rifle match, I most always shot the 300 better with iron sights, and often out shot the 300 with a M14. Why? not because the 308 was better then the 300, but it was me, I shot iron sights better, and I shot the M14 better then the M-70 (because the M-70 was a lot heavier and had more recoil.
Its not just target shooting. Lets look at hunting. We all know the 300 WM is an accurate long range round. Yet most hunters can shoot a 243 better then the 300. We can't say the 300 is less accurate then the 243, we can say the 243 is easier to shoot for most people.
So its not the caliber, its the shooter first, then the platform, then the ammo.