I have a hard time imagining that one company will be more courteous than any other, in general. There are pilots will all sorts of personalities at all of the airlines.
Agreed.
Say Joe Pilot who flies for ABC Express Airlines flies into KATL on Monday, having a great day, Flying the airlines newest and nicest airplane, cheerful on the radio, and so on.
Tuesday, he flies into KATL again. Only he's having a bad day, his wife yelled at him, he's flying the oldest airplane in the fleet with several items are inop and on the deferred list, he's just not in a good mood. He's snappy and short on the radio.
My point is, it's based on the individual, and even the type of day they are having, as to how nice/polite/joking they will be on the radio. I don't think you can say so and so's pilots are nicer on the radio.