Bryant if healthy. Bradley if he's not. Soley based on what we've seen on the field so far.
If you choose Bell you're going virtually solely on faith. JC RBs have at NU have had mixed results. From Rozier(maybe the only really good one) to Thunder Collins and Kenny Wilson.
To refresh the memory Thunder did have one good year and Kenny might have become a top level back but hurt his back lifting a TV if I remember correctly.