Really depends on where you fly most. If it's around the midwest/west, then you want an airline with a hub that's close by.. DL's SLC hub, UA's DEN and SFO hubs might be convenient.. If you're flying mostly to the south, CO's IAH or DL's ATL hubs might be best. And if you're headed to the NE, AA/UA's ORD is pretty good, and the combined HP/US should have some great connections from PHX through PHL.