A lot depends on what you're going to do with it, and what kind of features you'll want/need now and in the short/medium term future. In a nutshell, you need to consider the following:
1) overclock? You'll need an X370 or a B350 board.
2) maximum RAM? You'll need a board with 4 RAM slots vs 2 slots.
3) expandability/advanced graphics? You'll want something capable of SLI which means the X370
4) M.2 slots. Do you want a M.2 PCIe drive? Make sure it has one, or two.
5) Format. Which format? ATX? mATX? ITX? That determines quite a bit of the capabilities, and the ITX tend to be more expensive.
6) Who have you used before? MSI? Gigabyte? Asus?
So - it all becomes a balancing act of priorities.
My preference are Gigabyte and Asus personally. If you're not looking for a full blown tricked out board, you can get an Asus Prime B350-Plus board for about $80.00. Has M.2, 4 DIMMs, decent overclocking and power delivery.