Really? C is a favorite key of novice pianists because it is all white keys. But, on guitar, I prefer A, E or G because lots of open strings. But the key of C is easiest for anyone first learning theory, just as you say. (C is also easiest for brass--I played baritone horn and a little trumpet and bone in hight school band.) I suppose if you have any preference for key at all, you cannot consider yourself a pro!