According to the crowd at the comments in the review "Inexpensive 4x 2.5GbE Fanless Router Firewall Box" the HUNSN RS34g Micro Firewall Appliance with Intel Celeron J4125 AES-NI, 4 x Intel 2.5GbE I225-V LAN is the Best low powered pfsense box.

This box is the perfect all in one low power pfsense device. It includes a Celeron J4125 processor and four 2.5gbit nics. This allows for 2.5x times the bandiwidth compared to 1gbit when connecting to other 2.5gbit devices (such as recently released cable modems and certain laptop and desktop nics). In addition it includes RAM and SSD so no additional components are needed. It is will supported by pfsense 2.6x and above as well as other popular router/firewall OSs like OpenWRT and OPNsense.