Gotta look at two things:
1. Facilities. Obviously, the better facilities, the better feeder club.
2. How competetive the league is. You dont want to go too crazy here , your players need to be able to estabilish themselves as first squad choices. There are two schools : **** league but with possibility to play in european competitions, or solid league where they'd have to work hard every day. Personally i believe its best to send my young talents to leagues where they will be exposed to decent opponents on week by week basis, rather then sending them to **** leagues just because the club will get 3-4 games a year with quality opponents via european cups.
This is why i generally prefer Championship clubs as feeders. Decent opponents all year round, stadiums are generally packed so players will get used to playing under pressure ( i have no idea if it actually matters , but IRL it sure would - you just have to do your best if there's 20.000 people chanting your name. ).