I have home and family in UK which means I travel internationally regularly so realistically that leaves DL, AA, UA and the ST/OW/*A counterparts not WN etc

I got DM and am addicted to the status and extremely regular upgrades and OpUps
I love the Choice Benefits especially GUCs
Much nicer staff generally IMHO - the people really are great -as a poster says above, the culture and customer service are just great - I go out of my way to fly DL and will spend upwards of 200$ more per ticket to fly them - my wife will as well.
Better service and more comfortable in Y than AA or UA
Much better experience in IRROPS
When in F, I think the food is better than AA or UA. It is about even internationally but again I prefer DL operationally reliable