Delta. The only time you'll get me in one of their planes is for a one-hop to or from Atlanta, and even then I'll think hard about whether I can get on another airline. The number of patently absurd things they've done to me would fill quite a lengthy post, though.
Whenever possible (i.e., heading westward, domestic US) I fly Frontier. For other destinations, US Airways is my preferred option, mostly because their hub -- Charlotte -- is actually a rather nice and not-too-crowded airport.
I just flew with them this weekend. I'm pretty sure they canceled the flight I was on due to a small amount of passengers (there were only ~10 people waiting in the terminal). This resulted in a 4 hour initial delay, missing my connection, an additional 2 hour delay waiting for the next connection.
There were no people to talk to at my original airport, so I had to leave the "secure" area, go back to the ticket counter and wait in line for the one Delta employee to assist.
Whenever possible (i.e., heading westward, domestic US) I fly Frontier. For other destinations, US Airways is my preferred option, mostly because their hub -- Charlotte -- is actually a rather nice and not-too-crowded airport.