Forte. Just having him on the field forces defenses to bring another defender in the box, that results in a one on one coverage's for one or even both of the WR's on the outside. Plus he's a receiving threat so if teams commit to the WR's down the sidelines Forte can kill them underneath. I don't think either WR has quite the same success without Forte in the backfield to help draw favorable matchups.
Plus he is a monster statistically every season that he hasn't been hurt or dinged up. IMO he is the focal point of what makes the offense tick. I do think the running game eventually gets going when the Bears aren't facing a front 7 like Buffalo, SF, Carolina, or the NYJ. I'm pegging for Forte to unload on the Falcons, just my guess.