The best bra styles to pair with a milkmaid top are balconette bras and strapless bras.
If your milkmaid top features a square neckline, try a balconette bra. A balcony bra, or balconette bra, has a shallow cup and wide-set straps that attach to the sides of the cup instead of the center. Unlike a demi cup, which features cups that have a bit of an angle, a balconette bra's cups are typically cut straight across the breast, pushing the chest up rather than together. This style pairs perfetly with a square neck.
Another great option is a strapless bra. Strapless bras provide support and lift without any straps. Many strapless bras feature underwire, an extended band, or boning to keep the bra in place. Because milkmaid tops have wide-set sleeves that would the straps of a typical t-shirt bra, a strapless style is a good solution.