What is the best ground cover, perennial for a small flower box??

What is your location? What exactly is your version of a skall flower box? Do you mean a small planting area?What is your lighting?

I love pink or white or purple alyssum. It is a perennial, it fills out a little more each year, it grows over the side to make a nice, lush flower box look. It has names like "Carpet of Snow." Have fun!

I love mixed primrose, also called portulaca.

Sedum is a good filler, doesn't require alot of water, and comes back every year. They range from lime greens to reds so a nice mix of textures and colors.

phlox is nice and it will return every year. Not too tall