After Gruden, if no other ex-NFL Head Coach is available, I would look to guys like Charlie Strong or Malzahn.
I like to look at Head Coaches who have succeeded as Head Coach, and Coordinators. These guys were coordinators at schools that won National Titles, then dropped off after they left. That usually means that the Head Coach was getting the credit, when it was really the coordinators who were a big part of the reason for the success, not the head coach (yes, I know the head coach was at least smart enough to hire them.).
Say what you will about Mark Richt, but he has won a lot of games at Ga (114-39), after being a successful coordinator at Florida State. Since he left, FSU dropped off as well.
If I had to chose, I guess I would choose a former DC over the OC. I'll go with Charlie Strong.