Neither AA nor US are Amex transfer partners.

Of the airlines OP mentions, only CO is. And they're a perfectly reasonable transfer partner.

But the answer is that it depends on what kind of rewards the OP is interested in --- ANA is a transfer partner, just 63k for Virgin Upper Class JFK-LHR-JFK for instance. Air Canada Aeroplan offers unreal value in their first class mini-round the worlds to South Asia (as far South as Singapore) for 120k.

Avoid transferring to Delta...