Definitely no right or wrong answer. Also depends on what a player has naturally like what Ripkin Fan said. For my own son I'd say speed is the one that is the most difficult. I was a sprinter and form/technique can definitely be taught to improve speed. However, genetically some may not have been gifted with a lot of fast twitch muscles. Anyone can train to become an endurance runner, but you can't make just anyone into a sprinter.
His second tool that he is developing is his arm strength/throwing. I think this is just something that takes more time to develop and possibly there are physical limitations. If there weren't we'd probably continue to see faster and faster pitching, but at some point it becomes physically impossible to throw faster. Sort of like sprint speed.
So very interesting what everyone has commented thus far.