Do you consider the message on the bottom of the Virgin Atlantic business salt and pepper shakers — “pinched from” and “Virgin Atlantic” - branding? If yes that would be my favorite airline branded merchandise in my house (though technically may not be merchandise as they were not bought or sold - at least not by me). Second would be the Southwest branded bucket hat I picked up at the Cranky Dorkfest - it gets used more often than I should admit!

You used to be able to buy some of the Air France stuff in the lounges - but last time told it is just online now.

The Delta Museum has some cool stuff - in person or online: https://museum.deltashop.com/

Most of my airline themed purchases have been kids stuff - plush Air France and Delta planes bought in an AF lounge and the DL museum respectively. And we have both a Delta and Alaska kids’ airplane backpacks that were used for many years. My son bought a big inflatable Lufthansa plane in Frankfurt.