This is a pretty silly thread in my opinion. I think it has more to do with the person, not the country.
Yes, but where they are born makes a HUGE difference. If someone born in Brazil has the same ability as a guy who was born in the UK, the opportunities in Brazil to do BJJ/Muay Thai/Vale Tudo is much more than in the UK.
So I don't think this is a silly thread at all.
Lets have a look at ADCC 07 results, granted this is Submission Grappling, the same thing applies to MMA. Because you said that it's more to do with the person. When you look at these results, that obviously isn't the case.
Absolute Division - Robert Drysdale - USA
Over 99KG - Fabricio Werdum - Brazil
Under 99KG - Alexandre Ribeiro - Brazil
Under 88KG - Demian Maia - Brazil
Under 77KG - Marcelo Garcia - Brazil
Under 66KG - Rani Yahya - Brazil
That is one USA fighter as opposed to 5 Brazilians.
Why is this? Is it because Brazilian fighters have more ability than Americans? No OBVIOUSLY NOT. We are all human. It's just because of opportunities to do BJJ.
He asks which country PRODUCES the better fighters... That can be down to who has the better facilities, knowledge of the sport, coaches, etc. So this shows that where you live matters more than your ability.
I live in a small town of 5000, I could be the best fighter in the world, but I can't fight because there is nothing round here.