Even when flying international, my routes keep me on domestic airlines for the most part.
I fly mostly American because I'm coming from EWR/PHL/DCA mostly, and that's what covers the most routes.
Ive had others swear by Southwest but I've not been incredibly enthused compared to other airlines.
Flying has gotten better in the last 2-3 years since most planes have rolled out the newer, thinner seats that slide the cushion and the back. A decade ago my knees were always in the seat back and in pain. Now not so..
But just thinking about flights I've taken in the last few years...
International carriers:
Turkish airlines stood out for food and service, and I've found that the Turks are really good with kids, both flying on the airline and in person. But I'm not a user of any of the super luxury Arabic or Asian airlines.
Finnair was clean and pleasant, as was airberlin, which I think went away. British airways was fine but I had a decent seat on a 777, don't know thst it was particularly special.
For domestics, routinely flying in the cheap seats, Ive found that United is the worst - in the cheap seats there is the least knee space and the seats are the most uncomfortable. American is middle of the road, but the headrests don't go high enough. Delta seems to have more space, better seats, and the new cabins have better infotainment. As an example, on American (Im gold status there) , I have to use my own device to watch, and for example, I was going back and forth to the west coast a lot, and was watching Chernobyl. American only offered episodes 1-4. Fortunately Delta had episode 5 in the seatback tv. Southwest has ok seats, but that's assuming you can get a decent seat before a ton of others.
But for me it's all about comfort in the cheap seats. I'm not judging qualifications, staff, attendants, on time performance, etc.