I travel a lot domestically, fly on all the airlines. I hate to tell you this, but they all suck fairly close to equal, and who is better/worse changes every couple of years. Recently, I would mostly say American is the worst of the big ones. My current rankings would be something like this:
1. Delta
2. Southwest
3. United
4. American
A couple caveats:
Delta had been doing well then REALLY screwed the pooch with the CrowdStrike outage. If I can overlook one bad experience because of that, they are still probably the best currently.
Southwest is pretty good lately if you can deal with no assigned seats (and sometimes smaller leg room). Usually doesn't bother me on the assigned seats and I will normally buy the upgraded boarding, but once in a while I get tired of racing to the gate.
A lot of the discount airlines like Spirit, Frontier, you are rolling the dice and I would avoid even though sometimes they work out great. But of the smaller airlines, my current two favorites are Jet Blue and Allegiant, have had multiple good trips on each.