What is the biggest difference in quality you have seen in works made by a mangaka?

I am sure the art evolves after some time. Still, we are talking the quality in plot as well here.

Yentato Kabuki going from Black Cat to To Love Ru

i would say by yusa sensei
and she only ever drew manga and none of them got adapted but her story quality is good

The big three for sure, like Naruto, Bleach and OP.

wow demon and yeah you are right
we can always compare those managaka with their other works for sure