The biggest lesson I've learned in my 10 years of following prospects closely is that age is the most meaningful indicator of future success.
A teenager in full-season ball is almost always more deserving of a close look than a player in his late-20s dominating Triple-A. Younger players have greater learning curves than older players. They have more projection.
With the above in mind, I've scanned the minors and closely looked at every player who was a teenager as of July 1, 2014. The top 10 list below is purely based on numbers. No scouting information was utilized in creating it, aside from the usage of metrics that I have found to be the most valuable in predicting future success. If a player appears lower on this list than others, consider that his scouting profile may not yet match up with his production, or visa versa.
Did I miss anyone? Think my ordering is way off? I'm happy to discuss further on Twitter @AdamWFoster.