To my mind, it is Andrew Lambo. My reasoning for my choice:
Whereas James McDonald provides a good and potentially great power arm to the Pirates rotation, it is still a fact that the Pirates have already begun to stockpile and develop pitchers; over the medium to long term, pitching should be an organizational strength. This statement is true now, before the Pirates sign Taillon and Allie.
The same cannot be said about the minor league outfielders. This is the reason Lambo is important to the Pirates. He is young enough to fit in with Andrew McCutchen, Jose Tabata, Pedro Alvarez, Neil Walker and Tony Sanchez; he's also an outfielder who bats left handed and may develop the power the Pirates need in an outfield that will not often field a power bat without him or someone like him. Naturally, although the Pirates might need Lambo to develop into a quality major league player, he cannot continue to indulge his yen for recreational drugs. I also assume that Garrett Jones has peaked and that his performance will reflect that fact.
I've included a poll in which you can provide a quick response to this question.