I have been on hundreds of flights, both domestic and international, and can honestly say that there is no airline that I have hated. In fact, there are not many flights that I can think of that were unpleasant.
My least favorite flights have been with ATA and AMERICA WEST, though.
This is purely my experience and not a generalization of the airline, but my least favorite experience has been with ATA. I flew them this past summer SFO
on a 757-200. The plane was cramped, the food consisted of a tiny piece of shortbread, the flight attendants were cranky and frumpy and the clientele was appaling - the lowest common denomintor of human life. I felt dirty just being on that flight. But the ticket was cheap, so I guess I can't complain. I'll definitely pay more next time.
I have flown America West several times, usually on tattered old 737-300's. The flight attendants are unmotivated and frumpy. On the one flight I was on which served a meal - an A-319 from PHX
- the food was slop and awful. The inflight entertainment has been pointless trivia games which are shown via monitors. I just always get the feeling no one is motivated to be there and everything is done half-assed.
I would not consider any of these bad flights, but these are my least favorite airlines and experiences.