"Rank the tools from the easiest for a player to develop to the tool that is hardest to develop."
1. Baseball IQ - you need zero athletic ability, skills or gifts to become an astute student of the game...if you are using the correct source materials and understanding them correctly.
2. Fielding - but with one condition, you have to be in the right position. Wade Boggs was never going to be a SS. That's why no one tried to position him at SS. He was a third baseman and inferior at it at first. But, he put in the work and got a lot better. But there was never going to be work that would allow him to be a SS because he doesn't have the skills for it. If you are in a position suitable for you, then you can get better if you put in the work.
3. Hitting for power - assuming you already can hit then you can improve your power. It requires using the gym and getting stronger, learning to use counts and situations to look for a pitch to drive and learning to hit the bottom of the baseball.
4. Hitting - many great hitters are born but many others made themselves into good hitters. If you have the eyes, coordination and required mental processing skills, you can get better at hitting with proper coaching and putting in the work.
5. Throwing - you might be able to add 5 MPH. But, you're either born with a hose or not.
6. Speed - you can improve it but not all that much. Elite speed is something you are born with...or not.