In the past two years, I have flown AA, DL, UA, WN, G4, F9, AS, and HA as a non status traveler in Y. While they obviously all have different rules, seat pitches, IFE, etc., I couldn't tell any difference between any of them when it came to interactions with employees. DL does do a good job of running an ontime operation, which is going to naturally limit the number of "bad experiences" by travelers.