I finally did it and all it took me was a year out of my life! The goal? Catch every pokemon in "Pokemon Lets Go Pikachu!" and "Pokemon Lets go Eevee!" shiny, and then become the grand master of the Kanto region.
Pinsir is the one that sticks out in my mind. Its not necessarily the hardest one to get mathematically, but its definitely one of the ones that took the longest. I had to have found 10 shiny pokemon while hunting it before I finally got pinsir.