About a decade ago i would have said Brazil, no contest. As for now I'd say it's pretty close but I'd vote for Colombia, because Canela Skin
If you take aside countries who always have quite many top tiers girls (USA, UK, Czech Republic, Canada...) you'll notice that the porn industry has time periods and cycles : For a while a certain country will have no major pornstar then suddenly, suddenly they're gonna have 2-3 top tiers girls.
Take australia for example : Name a top-tiers, pre-Angela White, australian pornstar. Now you've got Angela who's arguably the #1 girl in the industry and you've got Samantha Bond who has recently risen to become a more become more popular than any other australian pornstar ever, except Angela and is considered as a top-tiers girls at the moment.