PermanentLiminality

Define best.

I like the Wyse 5070 extended for a router. Has a x4 PCIe slot for a network card. Not the cheapest, but very low power.

Any HP, Dell or Lenovo SFF desktop with 4th gen or higher CPU. Probably $50.

If you only need two interfaces you can go with the micro boxes and put wired networking in the wifi slot.

I think it is slightly high on price, but not too bad. Enough RAM/storage to get started, but expect to add more of both. Virtual machines eat more RAM so using containers (LXC or docker) will allow you to run more in 8GB.

Since you mentioned NAS, a micro system isn't the best as there is no room for a high capacity 3.5 inch drive.

I have a HP 600 G2 with a i5-6500 in a small form factor (SFF). It cost a whole $55 and has room for one 2.5 drive and 2 3.5 drives.