As a primarily coach flyer, all 3 major US airlines are basically the same.

Obviously people's experiences can be so different depending where they are originating and where they are going. Each airline has their own operational challenges. I definitely perceive Delta as being the best, but I doubt that is true. Being Atlanta based, I prefer Delta simply out of convenience, but sometimes the "Delta premium" is too much for me to justify. I believe that Delta is insanely good at branding and marketing and a lot of their reputation is based off this.
I have yet to have a horrible enough experience on AA or UA where I decide I'll never fly them again. I find a good combination of price and total travel time and make my decision based on that.