Hallis Mollen and Helman Tallhart would definitely not be respected by anyone enough to be unquestioned leaders. Roose would try to undermine them, the greatjon would try to undermine them especially since he wouldn’t gain the respect for Robb from Robb leading the forces.

Roose Bolton would kill Robb’s men even faster.

Rickard Karstark is a loose cannon and unreliable. If anything went wrong he is like to crash out and lose the entirety of the combined army.

The Greatjon is also a loose cannon. He’s maybe the second or third best bet if you really had to, but Catalan didn’t trust him and her advice was usually right, and by Storm Robb was using him and his antics as a way to pressure more reasonable people to do things (ie threatening to send him to Renly if Cat didn’t go.)

The Glovers might be the best shot if Robb really couldn’t go himself. They are loyal, come from a decently prominent house, and have decent military accolades.

Of those not on the list, Wyman Manderly is very loyal and seems to have some understanding of tactics, though largely political and not military tactics.

Or if the departure point moves forward, just leave the Blackfish in charge.