I really don't know, and since we just introduced 3 new chickens (including a cockerel) and culled 3 the new order is still up in the air. So far the old and new chickens don't really mingle much yet.
Before the big changes, I'd guess it was my silver laced wyandotte, Gertrude. Not that I ever saw her peck at any of the other girls or give them the stink eye, but when we were having feather picking issues she was the only one who wasn't missing any feathers. My leghorn (one of the 3 culled) sure fancied herself the boss, she liked to crow and on several occasions I even saw her mount some of the other hens and peck them on the heads. But I've also seen at least one of the other girls chase her off.
Amongst the new chickens, my blue orpington (yet to be named) was without a doubt the head hen. All she had to do was look at the poor little ameraucana girl and she'd run away. And while I've seen her back down from my roo, I've also seen her give him hell too. I'm really hoping that when all is said and done she will end up as top hen because she strikes me as a benevolent leader sort.