CC image from kla4067 on Flickr.
The Dodgers' Matt Kemp led all National League center fielders with 14 assists last year (standings). Does that mean he has the best center field arm?
Not necessarily. As the Orioles broadcasters pointed out a few days ago (at least I think it was the Orioles; I watch a lot of games!), once an outfielder has established a reputation for a strong arm, coaches and base runners become more conservative. Many of the runners he would have otherwise gunned out at second base or at the plate are now staying put at the previous base. Outfield assists would be a great measure of arm strength if base runners made the same decisions regardless of who's throwing, but that obviously isn't the case.
The same can be said about the number of times a catcher throws out base runners attempting to steal.