steve dave is the xpert, and that is up to you i suppose.
delta has nice metal (although older than average) and professional staff in my experience, but they just announced a downgrade in skymiles' value. this will probably happen with other airlines and has been discussed on here before somewhere.
i don't really like the southwest experience, but a lot of people do, and i think their points are pretty valuable (unless you want to go international)
can't speak to american/united other than i prefer leather seats to cloth seats