I've used a #4 Kreinik very fine braid. The designer button in the center is the perfect touch and matches the thread exactly. The button was a gift from a friend and had been in my "stash." for several years. It couldn't be more perfect.
I've used an awl to make a hole for the shank of the button. A 1/4" plastic ring secures it on the reverse side and allows the button to lie flush against the fabric.