With the recent move by AA and UA to eliminate mileage expiration, I decided to re-evaluate what I felt was the best FF program. My evaluation criteria of programs have included the following items:
1. Mileage expiration
2. Ease and cost of upgrades
3. Award travel availability (i.e. how easy is it to get a free seat)
4. Award travel mileage cost (i.e how many miles does it take to get from A to B) and restrictions

This evaluation is limited to AA, UA, CO, DL, and US as those are the airlines I have my miles with and who I fly 99% of the time. Most of my travel is domestic so that also influences my judgement.

1. Previously, I significantly downgraded programs with mileage expiration as I accumulated miles far faster than I could really use them. This hurt AA and UA. With the recent changes, mileage expiration is no longer much of a factor. CO, AA, and UA now lead the pack with DL and US in the back (they both require a flight every 3 years).
2. CO clearly leads the pack. Of the 5 airlines it is the only one that does nor charge for upgrades, provides upgrades on all fares, and upgrades for elite members are usually fairly easy to get. I would put US second since my success as a Preferred member has been much better than my success on UA when I was Premier (the equivalent status). I have not flown AA much recently so I don't know how easy it is to upgrade, but AA seems very similar to UA so I would rate both 3rd. DL has trouble with its L fares not available for upgrade so they rank last.
3. Award travel availability has been discussed elsewhere on FlyerTalk and the statistics discussed (% of seats that were used for award travel) as well as my personal experience and other anecdotal evidence suggest the following ranking: AA, UA, CO, US, DL
4. Most airlines have very similar award structures. There are differences here and there (for instance, you can get to Hawaii in coach on DL for only 30,000 miles while everyone else is charging 35,000). US and CO have moved to some reductions for off-peak domestic travel although CO's regular first class award is higher (45K vs. 40K) than other airlines. US has upped FC to 50K miles starting in September which is ridiculous in my view. Restrictions are very interesting. AA seems to be the only program that permits both a stopover and an open jaw on the same ticket. CO charges 35K coach to do a stopover while UA doesn't even provide the option. I haven't done a thorough analysis, particularly for international travel, but my impressions (combining mileage cost and restrictions) would suggest the following ranking: AA, CO, DL, UA, US

In summary CO still does well - they were my previous #1. AA's move to eliminate mileage expiration moves them up to CO's level in my view. UA would be next with US and DL clearly trailing. Both US and DL have a lot of work to do. Moving to total elimination of mileage expiration won't be enough to help these two. Both need to improve award availability, DL needs to get away from the L fare issue (sorry for beating a dead horse), and US needs to bring some of its award levels in line with the rest of the industry. I'd be interested in other views and comments on interational award travel.