I'll be honest, I havent really looked on my saves! I tend to be a LLM, so the top end regens dont really come onto my radar until ive been maanging for quite a few seasons!
Brazil and France for me. only diffence is that regens in Brazil can only be signed when they are 18+, which is very annoying as everyone knows. In France you can sign them without having to negotiate with the club. So maybe the regens are slightly better in Brazil, but I always look first in France |)
France generates the BEST ones. But argentina produces A LOT of very good players, i have 4 on my save atm, they are awesome, but the best regen i could find is french, one beast of a striker.
For me its Brazil and Portugal because of the shear quantity coming out of those two countries. But I always seem to come across one or two top, top class regens from the Scandinavian countries and for some reason they are probably my favorite to sign.