Everyone has their preference. If it were me, I'd avoid NorthWorst.
I fly American quite a bit, and very rarely do I have complaints. Unlike MattD, the political causes that American supports are ones that I agree with.
I've not always been a big fan of United, but I've never had bad service. I flew on one of their new A320s, and it was fantastic.
If you're shopping for price, you've missed the obvious answer: Southwest.