I gota say Delta but that is a biased answer seeing as I have only flown on Delta the 21 years I have been alive. Well technically Western, and then Delta once they bought them out. I still have some playing cards from Western.
The longest delay I have had on a flight was 1hour 30 min, never had an emergency, a "mean" F/A...nothing. Although I dislike the omlets they serve for Breakfast (nasty ass eggs, choose the Special K cereal instead) and the movies aren't the best, but they keep telling me they are not allowed to show "Showgirls" in flight so I guess I'm out of luck.
21 years of service without a hitch ain't too bad in my book.