The best Pfsense hardware is rather subjective. I think the ideal Pfsense box should deliver solid performance while being affordable.
A year or so ago, Pauldotcom did a podcast on building your own router, but the resulting beast was overkill. It was secure, but it cost way too much money and burned up way too much power. A router for home use doesn’t have to cost $400.
Start off with a motherboard based on a Celeron J1900. A Celeron 3450 would be nice, but right now I can’t find affordable boards based on that chip. The J1900 is fine for a router. Memory doesn’t matter too much. Get a couple of gigs, but don’t worry about loading up your router with 16 gigs. It’s overkill. Get name-brand memory like Crucial or Kingston.
Next, add a multi-port Intel or Broadcom NIC. These cards are cheap and offer much better performance than the Realtek chip the motherboard probably came with.
You can probably get by with any old hard drive for Pfsense, but I would recommend picking up an inexpensive SSD. A slightly used Crucial or Micron drive would be ideal. This will keep the device quiet and keep power consumption low.
You can get as fancy as you want with the case, but you can keep it simple too. A cheap and cheerful $50 case is fine.
So you can see, you don’t have to spend a fortune on Pfsense hardware. Recycle what you have laying around, and you can probably get going for under $200.