As stated, the best program depends upon your travel patterns.

My experiences as Gold with DL and US were good, however, I have been finding out that some of the benefits would have carried over longer with AA than the other two carriers. While my travel needs have changed significantly in the last 2 years, I have found that AA does not have an expiration on their 500 mile upgrades, while DL and US do.

US will allow you to have 800 mile segment upgrades reissued or exchanged for mileage credit, but DL just lets them expire, or so was my experience. With DL upgrades were available through CVG, but it was hit and miss through ATL. My experience with US was a higher percentage, plus US did not have the L fares which weren't upgradable.

The bonuses for each of the 3 were equal, if I remember right.

JMHO.