As others have said, it's a complex question that needs more information from you.

If you fly 5 times per year, you will barely make silver elite level. If you don't make it, the upgrade issue isn't important.

Do you have a favor airline. Some love AA, some hAAte it. Ditto the other airlines.

If you don't mind connections or if you are flying from LaGuardia, US Airways has a 20,ooo domestic award versus the others' 25K. Delta doesn't require a Saturday night stay for domestic awards. Jet Blue might have the lowest fares. Many forget about Midwest Express which has extra wide seating. Delta doesn't allow upgrades on cheap tickets.

Alaska Airlines ff plan isn't any good for those wanting upgrades or for those with high elite status. However, you can get miles from American, Continental, Northwest, KLM, British Airways, etc.