Jake Locker
I think they sign him and it will be Manuel v. Locker v. Cassel. And between his natural ability, him fitting perfect for Roman's style offense, more weapons than he ever had in Tennessee, and his career being on the line - he wins.
However, if Locker can't be brought in i'd say they should draft Garrett Grayson as the 3rd QB. In that scenario - I give the edge to Matt Cassel.