My primary carriers are UA (1K) and DL (Lifetime FO) and that suits my current travel patterns just fine. HOWEVER, UA is of course in bankruptcy and DL is teetering on the brink of bankruptcy. So, I'm in an earn 'em and burn 'em mode. If your goal is to rack up mileage in a relatively safe program in order to cash them in years from now for that trip of a lifetime, look elsewhere.

If your short-term goal is to score domestic upgrades, shoot for atleast mid-tier level in any legacy carrier program. If your goal is to score international upgrades, UA and AA are pretty good. DL/CO/NW are tough. If your goal is award availability it depends on so many factors, e.g. frequency, season, equipment, it's difficult to say which program is better at any given time.