The easiest transfer would be MEX though with a probable terminal change (or conceivably CUN and no terminal change). On your list PHX and to a lesser extent SFO on first principles would be better but depends on the airline used to some degree. LAS and particularly LAX can be congested for what is effectively an international to international connection. LAX on the other hand has the best facilities once you are airside for the BA service, in terms of the Qantas First lounge, other features of TBIT, plus the A380 service.

Not on your list is SAN. The lounge is a locally provided version of an Admirals, somewhat better than the norm, but the transfer experience would hassle free, and it's not a big airport, in fact one of the best run in the USA.

On the east coast, PHL would be my choice, it's one of the few airports in the USA with something approaching a Flight Connections service for international travellers.