In NYC DL flies to both JFK and LGA. It IS the world's largest airline!

Partners? DL does not need a lot of partners as they fly almost everywhere themselves. DL gives unlimited free domestic upgrades depending on status. DL has good service, friendly FA's and seat back entertainment on a lot of flights.
AA is an good choice if you go over 100k per year. They do not have seat back entertainment. Unless you go over 100k in travel every year, upgrades will be limited if at all. Not as friendly as DL but still friendly.
UA no way!
US eek!
CO avoided them at all cost, even as a ST member.
AS excellent!
But you really need to figure out where you will travel and where you will want to go. Then it gets easier to choose or spread.
DL or AA!