Personally, I would rank each country by their fighter's success.
1. Brazil: With champions in Aldo, Jacare, Shogun, Silva, Fernandez, Feijao, and Sandro, Brazillian fighters are right now without a doubt has the most dominant fighters.
2. USA/Japan: the reason they are both tied is because of the fact that they do have the second most dominant MMA world champions, but their all in their respective countries. (All American fighter are champs in the US and All the Japanesse fighters are champs in Japan)
3. United Kingdom: Though dominant in their homeland, their influence in the US has allowed them to showcase their high-level striking.
Honorable Mention: Russia, Netherlands, South Korea
based on "quantity of A-C level fighters total":
If we would only count the guys in top 5 in their divisions Canada would pass UK (0 guys left then) and be tied with Japan for 3rd place (again, based on #s only). Also notice that I am counting only A-C level fighters, if we include the cans from all the scrub shows across the globe (specially japan) it would be harder to rank the countries.
Based on "amount of UFC belts held right now":
1. Brazil (3)
2. USA (2)
3. Canada (1)
Someone less lazy can do the belt math including the lower tier organizations too, I find them irrelevant for this point though as none of them outrank the respective champion of same weight class in ufc anyway.
Simply put USA dominates by pure numbers of fighters, but the most competetive divisions are ran by brazilians and a canadian right now. Also if we held some sort of "mma olympics" tournament for each weight class, I believe Brazil would come on top not only with most gold medals, but by most medals overall too as they would yoink atleast bronze medal from any division excluding LW.
To be honest, if there were MMA Olympics, Japan would dominate in the lower weight divisions (BW,FW and LW), Welterweight is the US hands down, Middleweight is a bit of a toss up between the US and Brazil but I think Brazil might take it by a hair, Light Heavyweight is Brazil by a long shot, and the Heavyweight division is definitly in a mix with the US, Brazil and Russia