If you are not primarily a domestic road warrier, then CO is worst.
I think that there is no correct answer. Each program has its strengths and weaknesses, I think smart FT'ers find a program that compliments their individual flying profile.

For instance, I used to be loyal to CO. But when my flying patterns changed from domestic to international, I had to adapt and change to AA.

It seems though that Delta's big saving grace,among its unhappy elites, seems to be lounge access. This gets a big yawn from me. Big deal. So what? AA (as well as UA for that matter) allow access for international travelers.