I have a strong preference for Southwest. They tend to be quite reliable, flexible, and cheap. Given the option, I'll fly them. Only trouble is they don't go to a lot of airports. My Southwest preference means that my experience with other airlines is fairly minimal.

I flew Frontier to Denver and back. They didn't quite have the Southwest vibe, but I don't recall having any real problems. Although I tended to dim the seat monitor because I didn't feel like paying the $5 to get a range of channels and I found it a distraction.

The main other airline that has burned me is Northwest. They've just had a few too many problems. Admittedly that observation is a decade old so it might be out of date.