I formerly used to have a stack of cards that was effectively useless, 5,000 miles in five accounts gets you nothing. This was before the alliance alignment that occurred a few years ago. Since then I have seen the light and abandoned all but one card in each alliance, UA, DL, and AA.

AA is the only option for oneworld, you would have to be somewhat crazy to choose US in Star so UA is the obvious choice there. For Skyteam I would choose based on geography as all seem to be different but equal in their benefits.