I voted NW because they're the most vulnerable right now. There isn't any WN presence to speak of in their territory. MSP and DTW have pretty decent O&D as well (especially the growing MSP).
UA: DEN is already covered by F9, ORD is in the WN MDW catchment, SFO is in the OAK catchment, and IAD is in the BWI catchment.
DL: ATL is already covered by FL, DFW is in the DAL catchment, WN already won in SLC...maybe CVG?
AA: ORD is in the MDW catchment, DFW is in the DAL catchment, MIA is in the FLL catchment, and JFK is covered by B6
CO: IAH is in the HOU catchment, CLE isn't worth going after, and EWR is sorta covered by B6's JFK hub
HP and AS: The battles are over.
FL and B6: those battles are the last ones WN wants to get into right now.