The one that gets worn more than any other piece in the drawer. Not because it's special occasion. Because it's the romper that fits right, feels soft, and goes with everything else they own.
100% cotton with a round collar and button fasteners that make changes fast. The cotton is lightweight and breathable for warm days, and soft enough for sensitive skin without any tags or rough seams to worry about. Five colors, so you can have a fresh one ready while the last one's in the wash. You'll run them through the laundry three times a week and they'll keep showing up the same way.
Gray, yellow, pink, white, blue. Every color works on its own and layers under sweaters and overalls when the weather turns. The everyday piece that quietly becomes the most important thing in the rotation.