You have a long flight ahead of you, not to mention going through airport security. What’s the best choice for clothes? Layers are always good. You can add or delete depending upon the temperature in the airport, on the plane, and your final destination. Opt for an easy to remove sweater or light jacket.
Loose fitting clothes are a good idea, too. Since the cabin is pressurized you want to be able to get up and walk around without tight clothes making your circulation even more strained.
How about your footwear? You have to remove your shoes. Make sure you can easily get them on and off. Consider comfort here as well. If you will be walking around the terminal or will be walking when you deplane, you should wear your most comfortable shoes.