This will largely depend on how much (read: bandwidth) you want to route. I used to be a big fan of the PCEngines ALIX boxes (https://openwrt.org/toh/pcengines/apu) because you could run pretty much anything on them, but with big gigabit connections nowadays, they're less well suited.

If you're looking to route the lower end of things, you're likely to be fine with an SBC like the above. If you're looking for more, then something with Hardware offload is worth looking out for. With OpenWRT, you're likely to be looking at either a fairly meaty X86 for gigabit, or an off-the-shelf for which it supports HW offload.