Any route away from a major population base
Disclaimer: I have no experience with Delta.
However with AC, any route / hub away from a major population base works best. Toronto YYZ is the worst whereas Montreal (YUL), Vancouver (YVR) and Calgary were much more favorable.
To interpret this into your situation, I would stay away from ATL and MSP since all of the Mid West and South East USA flights will be connecting through there. Doubtful that people will go backwards to SLC as a preferred routing.