I'm sort of locked into United because of their hub here at IAD. It's pretty good overall with lots of direct flights to places I go, but it also...means I'm always on United. For years we were big on American, but they offer less options for direct flights, so we rarely fly them anymore. Delta, which is generally rated higher, is even tougher for us as an option.
How about you? Good hub nearby? And happy with the airline(s) to choose from?