wish i knew where i read it, but i remember seeing that ISA browns lay 60% (or more; don't remember if that was off ALL eggs, or just brown eggs) of the eggs that are in the market. they are a RIR hybrid.
Leghorns by far or at least my Grandma says so and she has been doing chickens for eons!!! I always trust what the old timers say cuz they have been there done that!!
My production reds from TSC. Out of 6 hens, I got 6 eggs a day for almost a year & a half. And we got alot of eggs that would barely fit in the jumbo cartons.
They were egg laying machines.