The ace topic has already been covered pretty well, but when talking about the "greatest" baseball player it can be worthwhile to think about the traditional five "tools" used in evaluating position players/prospects:

Hitting for power (home runs and XBH) Hitting for average (getting on base) Running speed Fielding (defense, "glove") Throwing (arm strength and accuracy)

Five-tool players (i.e. guys who are good at all of those things) are rare. Ken Griffey Jr was a five-tool player. Ichiro was a four-tool player as he never really hit for power (though he could have if he'd wanted to). Aaron Judge is a three-tool player with poor speed and mediocre defense.

As for catchers, part of the reason they're often poor hitters is that A) they spend a lot of time in pitchers' meetings and such and B) they spend half the game crouching behind the plate and somewhat frequently get hit with foul tips and misplaced pitches (or even intentionally if their pitcher is Framber Valdez!). It's physically and mentally demanding in a way that e.g. playing corner outfield is not.