The question was which OF had the highest ceiling and I don't think it can be much of a debate that the answer is Starling. We can debate how likely he is to reach that ceiling, but most prospects do not reach their ceiling. I agree with that Starling has a long way to go to reach that ceiling. I would say that in fantasy, the prospects with the highest ceilings tend to be difference makers. Difference makers are the players that win leagues in my opinion. While taking prospects with high ceilings that are further away is more risky than taking a "safe" prospect with a lower ceiling, the high ceiling prospects can become truly special where as safe prospects are solid.
It is really two schools of thought of which I fall on the high ceiling side. Starling may take a few more years to develop than Taveras will, but the ceiling is higher and of course we should remember that many top prospects hit well in AA and AAA while taking a long time to adjust to the majors (Brandon Wood, Ian Stewart, Travis Snider, etc.). "Safer" prospects are hardly a guarantee to perform at the major league level either...but they do get there sooner sometimes.