What's your single favorite piece of travel clothing?

Just curious, I'm doing a wardrobe refresh! What single piece of clothing was the best addition to your travel wardrobe, and why?

For me, my Columbia windbreaker/rain jacket that folds up into itself was a game changer. It insulates well, takes up practically no space, keeps me dry, and even looks good. Pockets galore. I wear it most days under 70 degrees

Whats yours?