If OP specifically wants to connect, DL is clearly best option of those 3.

OP also mentioned international flights: IMO, DL has the best J class internationally. I think US is better, and then CO and DL are comparable, both offering options some might like over others, and then UA and then AA. So if it's between UA J, then go for DL. If it's CO J (BF), then it's more of a question.

Why do you want to build up status quickly? If for upgrades, then again, go for DL. Otherwise, as of now, before anything further is known, UA probably has the best program for high level elites, definitely better than AA.