Hm. I've flown southwest a bunch for business and pleasure, and ... maybe I'm a sucker, but I generally can't find any fares lower than Southwest (with baggage taken into account), I like the lack of classful seating, since I don't have EliteWTFOMGMVPBBQ status and barely any corporate travel (in the tech industry) is first or business class for anyone without 'Executive' or 'Chief' somewhere in their title. I think the service is fantastic and polite, the seats are generally a bit more comfortable than many of the cattle-class seats on legacy carriers, and they historically haven't screw me around with fees, weird-ass fare classes, or anything nasty like that. Granted, my experience with southwest is changing as they start to add paid services... but that's how it's been.
I've had a similar experience with Virgin America, and I preferentially fly them anywhere I can, even though they're airbus. Yes, they have a first class cabin, but they have inexpensive upgrades for available seats, the first class tickets are reasonable to purchase outright, and there aren't a billion business douches flying on them.
All that said, I'm going to start putting a lot more time in on AS, for obvious reasons, so I'm getting to know them a bit better. So far, they seem like a decently classy organization. And I've had several good experiences with DL—especially the time I bought a first-class ticket outright to ATL—and the other majors...but honestly, in the end, I've enjoyed every trip I've taken on Southwest, and I've never felt "dicked around" by them. Historically, they've always been friendly to me from front desk to gate agent, cabin crew, etc. Also, their mileage program is the only one useful enough for me to have been able to use, though I hear they changed the rates recently.
If all that makes me a "sucker" who's bought into the "brand" being sold, then hey, oops. All I know is that I always check all the fares available when booking a trip, and SW consistently has been the cheapest, especially with baggage taken into account. That's not the only criteria—I generally wouldn't fly RyanAir, for example—but I think they have historically had a good product.
~Fox
PS. ... of course, we all know that K5 is the best.