I find all of the domestic airlines are pretty close to equally bad. Is Delta a little better? Yes, but they cost more. I've been on Jet Blue a bit lately, and think they do an above average job. Southwest used to be really good on cost, but it isn't anymore. I just take whatever airline hits the sweet spot of cost and schedule. Being in San Francisco, United has a hub, but I fly American, Southwest, and Delta plenty, too. We've started using Sun Country to get back and forth to Minneapolis. The pricing is a little jenky, but it's less than United or Delta (the other direct options). The scheduling is much more limited, so still happy to use United or Delta when I need a better schedule.
I find the major international carriers generally have a higher level of in-flight service, but that doesn't mean always, and better in-flight service doesn't mean better service overall. I would much rather be on British Airways, Lufthansa, or Air France than United or American (haven't flown Delta internationally), but I'm still going to pay attention to the cost. We flew New Zealand Air to get there and back, and it was much nicer than I would expect United to be--but it was still coach on a long flight.
