I've been moving towards Northwest. Since I travel pretty much anywhere I wanted a wider network than US and AA is usually too expensive. You have to connect to go anywhere other than MSP or DTW but those are much nicer hubs to hang in than CVG or ATL.
AA might be another but their fares seem to be higher and I felt better about NW's long-term stability.
If you're a PM you ought to be able to get comped status from NW. Search the DL forums for more info.
-cwk.