I believe some countries have overall a better poker field and if I have to make a top 3(I play mostly on Unibet where we can't know our opponents' info and we can even change 5 aliases/day... and my 2nd site is 888poker, so I'll pick my leaderboard from here), I'd go with:
1. Brazil - good aggressive players that aren't afraid to make the best decision they can and even don't mind gambling at any point of a game, regardless of the stacks/buy-ins... IMO this is the top 'poker country' ATM and it's not even close...

2. Ukraine - very good players overall, but at least at my stakes the majority are a bit more passive (at least compared to the players from Brazil), probably TAG style is the most used by the poker field from this country and for some reason- the most hero calls I've seen at my tables were made by players from Ukraine- well deserved 2nd spot...
3. like it was mentioned above, on 888 there are quite a few players without a country flag(different countries, but IIRC mostly Russia, Belarus, Hungary...), so I'm a bit more cautious when I'm in big pots/tough spots vs. players with '888 flag'...