This year? Maybin. Doesn't have much in front of him, Marlins are committed to being bad, and even if he struggles to get on for awhile, he will run a bunch.
Unfortunately for Rasmus and Bruce, their respective managers don't like younger players, so they may not get ample opportunity to play this year.
For the future, I think Bruce will be the best of the bunch. I like Maybin slightly better than Rasmus, just because I think Maybin has higher 'upside'. If Maybin reaches/exceeds potential he will be the best. I don't think he will, but he has a chance.