I think "the strongest family" is always a relative concept.

On one hand there are the members, their potential to violence, their earning ability and their head count. Then there's the boss/administration, their ability to make money and their ability to maneuver with other bosses. For example a family interest in a something like the windows case isn't really a family interest, it's the administration that gets the money. It seems that Carlo (with the his alliance with Tommy Lucchese) was able to, if not dictate, at least strongly affect the policies ruled by the commission, and I suppose win a lot of sitdowns. At his prime at least, I guess Castellano took over more and more as he got older. So is the strongest family the one who has the most influential boss in the commission, richest boss, or is it the family with most hitters and earners?

For example the Gambinos would be a weaker family under Junior Gotti than it would be under Carlo Gambino, even if they had the same amount of killers, earners and overall members.