Personally, I've liked Delta. I've had one instance where they changed my window seat to a middle because they changed planes and eliminated the row I was sitting in. But, I've taken advantage of their change policy (if departure or arrival time is changed by 60+ minutes, you can change your flight for free) multiple times to better times. One time, we had even booked an airport 90 minutes away for a cheaper flight. They actually called me and asked if I could go on an earlier flight, I was able to move to our "home" airport (30 minutes away) at decent times for no charge.
I like that even in coach you get free drinks and a snack (granted, not much, but it's something), even (most of the time) on short flights. I flew American once (not booked by me). And on a 3 hour flight, the only thing they would do is give you a cup of water.
I like Delta's entertainment (can stream movies/tv shows to your device), and you can pay for internet and browse the web if you'd like. I think SW does that now too, I don't know about other airlines.