What is the best laying chicken
Likely one of the Hy-Line hybrids. They are used on egg farms all over the world.
http://www.hyline.com/aspx/products/productinformation.aspx
http://www.hylinena.com/aspx/salesmanagers/salesmanagers.aspx#
I don't think they sell a handful of chickens, more like 10s of thousands.
There are other producers of commercial layers that produce very well.
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As someone else said, sex links are good.
As for a pure breed, leghorns for white eggs and RIRs for brown eggs.