Argentina probably has the best quality/price ratio, but work permits can be crappy. Croatia has a pretty good regen rate on my saves, eastern european nations would be my pick.
Although Brazil and Argentina generate tons of prospects, I was amazed that there's quite a bit of talent in Africa as well.
You might say that Africa is a bit too general, but as I don't play any african league (and don't suppose you do) there aren't a lot of regens (or better said, non-grey players), so scouting is quite easy and quite often worth it.
Opposed to this, I have both Brazil and Argentina playable (yes, I am indeed regen greedy ), so with the amount of players scouting has turned out quite tough there. Although I must say the scouts are actually quite accurate as opposed to previous versions...