Choose the airline with the best schedule, where they’re equivalent with the best reliability, and for regular coach comfort go with Southwest. Since I need to work inflight pretty much all the time I prefer American or Delta, United still isn’t an option though if I lived on Staten Island I’ve fly out of Newark and sacrifice productivity.
No airline is all things to all people, no airline frequent flyer program among those four carriers is a real differentiator and reason to go out of your way to fly a specific airline, but it’s useful to stick with one carrier and earn status where you can. Your miles will be earned through things other than flying and there flexible points transfer programs are best.