This is my personal ranking of airlines i have flown over my 3 years and nearly 400 segments. I am leaving out AA since I have less than 6 segments with them, same for Spirit and Alaska.

1. US Airways - Hands down winner
2. Midwest - Wish they went more places
3. CO - Possible #1 carrier if not for EWR
4. ATA - Favorite discounter
5. AWA - Heard stuff, uneventful & polite
6. NWA - Take out DTW & they move to #3
7. UA - Nothing positive except US codeshare
8. DL - Earns last only because the US code share pulled UA up one position.

This ranking reflects my experience only and is based primarily on my perception of employee attitudes and performance when things go wrong. That IMHO opinion is where DL & UA fail miserably is when the manure is on the fan.