There are 90 teenagers playing in full-season, affiliated baseball right now. None of them are 17. Nine of them are 18. And the rest are 19. Only 15.6% of them are playing above Low-A. You can view more teenager data here.
Numbers heavy, but with some of my personal scouting factored in, the list below is meant to be a conversation starter more than a definitive "I'm all knowing" set of rankings. Comments, especially from people who have seen players on the list in action, are welcomed.
Were he still in the minors, Bryce Harper would be firmly entrenched atop this list.