Great read, as there are certainly a lot of parts that go into being a catcher. Just to simplify things, preventing stolen bases, footwork and positioning behind the plate and calling a game/working with a pitching staff. I've read a similar article from someone on Fangraphs as well, so its nice to see another one.
That being said, my list would be Mauer at the top for sure. The second tier is a tie between McCann and Yadier Molina. McCann is the better hitter by far, but Molina is considered the best defensive catcher by many.