For me Delta has been the one that is garbage lately. It used to be my favorite but the last five flights have all had something go wrong. When I requested proof of delay for travel insurance it took them ages to send it over. Tbf though a lot of the issues might just be related to their hub being in Atlanta. I suppose it is hard to make such a gigantic airport efficient but wow do I hate Atlanta layovers (and being stuck there overnight when something goes wrong).

Only one I've never had issues with is Alaska but Southwest is usually good, plus their boarding procedure is by far the most organized. Seriously, why do so many airlines just make it a mad-dash by group?