FF miles aside... I still think DL is top overall. As a Delta Plat, and a United Silver, I had the pleasure of flying both, in both classes. Even flew AA in coach after DL booked me on AA direct after a DAL flight was really late.

In first class, I think United and Delta are both terrific. I cannot speak for AA. However in coach... this is where the differences are.

On the whole, I have not had any coach trips on UA, nor my single AA flight where the flight attendants were anywhere near as nice, friendly, or attentive as the average service on Delta in coach. Until I got PM status on Delta, I mostly flew E+, and short of one flight, overall the DL Y experience is far above and beyond anything else domestically. I would even go as far as saying my average flight on Spirit, had friendlier staff than United in coach.