I recently wrote a short article concerning this exact question
I think Gallardo is clearly the frontrunner. We have seen in the last year or two what top pitching prospects can do coming out of the minors, while the hitters have been experiencing a more significant adjustment period. I've always believed that a pitcher has a bigger advantage than a hitter when the league is "unfamiliar" with them.
I don't see why Gallardo can't come in and put up an '06 Cole Hamels year right away. He's every bit the pitcher that Hughes is, though Hughes is in a better fantasy situation in terms of offensive help and the need for him in the rotation. That said, Gallardo woud be in a weaker league, a weaker division, and would have a good young offense behind him. I can easily see him being the best pitcher called up this year.