Depends on what you mean by pretty good quality Black Copper Marans probably lay the darkest eggs. But I am partial to Wellsummers because they lay pretty speckled eggs and lay more then Marans.
I think Wynette has fabulous Marans eggs. (from her pics) They are incredibly dark.
Bargain also has some Copper Black Marans, although I haven't seen pics of her eggs, she has beautiful birds. I hatched some of her Welsummer eggs, and they were nice and dark. (clay pot color) Some were speckled and some not. I hatched out 2 pullets, so I can't wait for those nice dark eggs.