I'd go Heyward, Montero, with Stanton well behind. Stanton's contact rates are terrible. It's incredibly rare to see a guy strike out that much in the minors and become an impact hitter in the majors. Other than Ryan Howard, no one has done it. Keep in mind I'm including guys like Adam Dunn and Mark Reynolds. Those guys struck out far less often in the minors than Stanton does.
Regarding Montero's future position, far too much is being made out of it. No, he'll never be a good defensive catcher. In fact, he'll never be an average defensive catcher. He'll probably be a bad defensive catcher, but his bat draws comparisons to Miguel Cabrera and Albert Belle. Mike Piazza was a terrible catcher, and I've read scouts who say that Montero is much more athletic than Piazza and should be, at the very least, as good a fielder. If Montero's bat is truly an elite one, he'll be an incredibly valuable player behind the plate.