ATL, MSP, DTW, and SLC are the major hubs. MEM and CVG are the minor hubs with the greatest chances of upgrades. However, good luck finding flights through these hubs (particularly if you are PDX based).
Also good luck finding flights with first class cabins... (A number of flights out of the smaller hubs are in CRJ-200s, I had one flight booked through CVG to MSY, which was originally booked on a 700 or 900, but was changed to a 200...
).