Current AA Plat; former DL Gold M and CO Gold.

AA is the best bet overall. DL gives out more upgrade certs, but am only able to use them on a small % of flights flown in Y. See no reason to sit in DL's uncomfortable Y product when can be reasonably comfortable in AA Y. And, when I feel like upgrading on AA, there are no fare issues. DL and CO also have overly restrictive intl upgrade policies.

The only thing I saw as better with CO and DL are their lounges-- CO President's Club is especially nice. But I have a lifetime PC so use that feature but fly AA where it's convenient.

Try claiming an award on DL or CO and you'll see another difference. This week I've wasted many hours trying to get 2 B-class seats to Europe this October through DL. Might as well try booking a flight to the moon. AA got me 2 seats on a decent itinerary with minimum hassle. Experience with DL by contrast taught me a lesson- little point in flying them cause they don't honor their commitment. CO is just as stingy.

The other area where AA wins over DL and CO is scope of their system-- AA is almost as extensive as UA-- DL and CO are much more limited.