If you mostly care about good award redemption, United, American and Alaska are all good choices. United, in my opinion, is still the best overall with the most flexible routing rules, Star Alliance partnership, and no fuel surcharge on any partner but the premium cabins got pretty expensive with the recent devaluation. Delta has a very bad mileage value. Alaska is interesting as it allows to collect on Alaska but also on Delta and AA but has a limited number of international partners.

If you don't care about status, then collect miles with all the plans. As long as you earn or spend miles every 18 months, the miles do not expire. You are then not limited to one particular airline and if you mostly travel on your own dime, it can save you quite some money. On the other hand, status gives some nice benefits which may be quite valuable (and addictive ): free domestic checked baggage/ extra baggage allowance, free preferred seating, priority boarding, lounge access on international travel, occasional upgrade, lower/no fees on awards or same day changes, etc. But for obtaining and retaining status you have to be loyal to one airline/alliance in great majority of your flights so it has some opportunity cost as your preferred airline may not be the cheapest. Only you can decide how valuable these perks are because everybody values them differently. Some of the benefits can be obtained easily with an airline credit card so an option to consider is to be "statusless" but to have credit cards in all major programs, or concentrate on one airline but to have a credit card for another one for occasional travel on the other airline.

There are also some foreign airlines to consider. In Star Alliance, Aegean, Asiana and Turkish have easy qualification rules for Star Alliance Gold (*G) but *G gives only some of the benefits that are available for United premiers and award redemption may require paying expensive fuel surcharge for some international travel.