Delta has been making pretty interesting improvements to international flights these last few years. For instance international flights have had free liquer and beer for quite some time now! yeah so delta is not too bad in coach. Now they are making some new changes , adding more meal choices , giving icecream as dessert , an amenity kit given to passengers . So if you have to fly on an american airline (low service levels) then i would choose delta over the rest (unless you can get an upgrade or award ticket on the other ones).
Delta is also pretty good on the costumer service side giving pretty generous compensations when something goes wrong. My delayed bag got me around $200 in expenses (to buy clothes and shampoo and perfume and such) so delta cares about the service side. My flight was once delayed by a few hours and i got 10k miles for that (i value it at around $180) so nott bad. I also had a year where i got bumped (accepted compensation to take other flights) a lot so i made around $2500 that year in free delta vouchers (which also explains why i got delta diamond status). So fly delta ! if you want to .
How to Free Travel 