It depends on the player, I am pretty good at call of duty and have learned much from modern warefare. I don't think it depends on the country, but it would obviously be USA because they have a bigger country, more people and which means there should be more good gamers than the UK.
It also depends what gamemode you think is best, I think search and destroy and the hardcore version is much more realistic so I tend to play that more. Others might have different opinions on what gamemode determines how good a player is, this is me in search and destroy;
[ame="
https://www.youtube.com/watch?v=tZY9JFDRyV4"]YouTube - quality test[/ame]
If you want to see my skills add iSpeeDD on live.