Taylor will benefit from the best o-line of the group, and historically productive running game, while Foster and the Carolina running game has been mediocre the last few years, and GB has probably the worst o-line/running game.
So the next thing to consider would be touches. All are highly likely to be in committee type of situations, although, if you read Yudkins spotlight on Foster, John Fox typically likes to give his starting RB the bulk of the carries, and rarely splits carries much between two backs mid-game. I think the average # of carries per game for the primary back was over 20. Foster is the starter right now, and I think he starts the season as such. If he doesn't get hurt, he very well could end up topping this group, but that's a big IF.
Although I'm not high on B. Jackson, and I think Morency will get the majority of the carries, I don't think either GB back will have a lot of value, since they haven't shown the ability to run the ball very well in recent years.
It's almost a tossup between Taylor and Foster for me, but I think I might be leaning more toward Foster, only because I think he has the better chance to see more carries. I think that Peterson will take over the majority of the carries a bit faster than many are predicting.