This survey says American Airlines has the worst customer service of the major domestics. And it says Delta had the biggest turnaround. I still think Delta is a flying sardine can with crappy seats, no room and lots of fees.
Worst Domestic Airlines
We've been restricting ourselves to JetBlue and Southwest. I did have a rather nice experience on Virgin America but they do not fly nearly as many routes in the US as I'd like (this one was Vegas to NY - had to take Delta on the outbound - ugh).