First things first - ranking prospects is both incredibly difficult, and incredibly subjective. How do you value stable predictability? How much emphasis do you put on variability? Does proximity to the big leagues factor into your decisions? If a player is incredibly raw, yet incredibly toolsy, do you rank them as a better prospect than a prospect who is what he is with little additional upside?
Hell, do you rank players as prospects if they’ve managed a cup of coffee at the big league level already?
In the nebulous world of prospect ranking, the folks over at FanGraphs do their dang best to create a formula to evaluate things, throwing all sorts of statistics and scouting insight into a blender that spits out a future value smoothie for us all to peruse.