Most people would say that UA has the best bang for the mile. So good though that starpoints transfers to UA only get you half the miles vs. transferring to DL, US, AA. ie 15k starpoints would give you 7.5k on UA. 20k starpoints would only give you 12.5k on UA.

With that constraint, AA would make a lot more sense, as might US (if you are looking for a partner star alliance redemption, for instance).

Redemptions to particular international carriers might make sense too depending on where you're planning on going. But I'm a lot less familiar with that space.