I’ve flown with a bunch of different airlines and really do not have a favorite mainly because I’m not fond of fixed wing commercial aircraft. That said, if it counts I REALLY loved a good parachute blast from an Air Force C-141. I mean, if I have to be crowded in like a sardine it’s comforting to know that sometime in mid flight I’m getting off.
Our favorite is Frontier. We fly that airlines out of DIA (Denver International) to Las Vegas. We especially like the Rock Mountain wildlife picture Frontier has on the tail of the aircraft.
I have flown many airlines all the way back to before commercial jet airliners, and, like @Beth Gallagher, did a lot of business travel once. Since moving to Alaska, Alaska Airlines has become my overall favorite. We have to go by air most of the time, as it is impractical or impossible to travel around this state or to the lower 48 by any other means. At one time, my favorite was Delta, then it became American, but both of those have fallen by the wayside in the service department. Alaska was near my boycott point after the episode of an Alaska flight attendant siccing the FBI on a customer she had a disagreement with, but I have heard of no repeats and I don't know if she is still employed by the airline after the bad publicity it garnered. I recall going to a lecture by the one-time CEO of Southwest. He told of being attacked by both Delta and American when it was a start-up. He came up with the idea of giving away free alcohol on all fights to customers over 21 years of age. That ended the fight, as Southwest gathered all the business travelers and Southwest became the largest alcohol vendor in the southwest U.S.