Three catchers on the team, one is super fast, with a weak arm, and tendency to drop fast pitches. One has a bullet for an arm, the best receiver of fast pitches but he is slow and his blocking suffers because of that, and one is medium speed with a decent arm but can't make a quick decision to save his life....who is the "best" catcher?
The one that can hit.
Well that's not REALLY fair...technically speaking the bullet for an arm, best receiver, but slow kid is the #4 hitter.
BUT...he can play 2 other positions and get his bat in the lineup...also the medium at everything kid bats 6th (also plays 2 other positions), and the super speedy kid (he is only that position) bats 8th...they ALL can hit....who's the "best" catcher? What makes a catcher "The BEST"?
IMO, the primary skill for a catcher is the ability to block. Blocking is unnatural and takes work. Good blocking solves a lot of issues -- it prevents runners scoring from third or otherwise advancing, it protects the umpire, and gives pitcher's confidence.
After blocking, then receiving. Are strikes staying strikes? Does he give the umpire the chance to call a borderline pitch a strike.
Third would be Pop-time and accuracy on throw downs -- not just arm strength. At the lower levels, a lot of things need to go right to catch someone stealing. It is not all on the catcher. The pitcher needs to be quick and the ss/2b has to be awake and be able to catch.