In a mATX form the best board will be one with a B450 chipset. But that hasn't been released yet and to my knowledge there are no mATX boards with an X470 chipset so you'll have to wait possibly till August for B450 release if mATX is your dream.
If you don't mind paying a premium you can find X470 chipset boards in mini-ITX form. It's bit of a waste, though, since there's no way you can SLI with only one PCIe slot and multi-drive arrays are kinda hard in those tiny cases. That means many of those extra PCIe lanes are unused; but you can get one with WiFi if that's a thing for you. If this suits your fancy then just look for any mini ITX X470 board and select based on price and feature set. There're not that many and you really can't go wrong.
If you have radical needs, like processor and memory overclock potential, things might get different though.