My personal suggestion would be anything you have lying around your place that has supported graphics. Graphics are always the big end-user pain point for any OS.
Protectli [0] has a bunch of systems that should meet any number of price ranges and network needs. I've not personally tried OpenBSD on them, but I see nothing that should cause any problems. They're also small and fanless systems, which I really appreciate. Also usable as general purpose machine, so not limited to simple networking.
A router doesn't need a ton of power. My router is still a single core. There hasn't been a need to update it because my internet connection is only 50 megabit. If you have a lot of users and heavy traffic, then you will need more, but if it's just you and the WAN, you really don't need much at all.
Honestly, any off the shelf or eBay x86 mini-ITX board is fairly well supported. Probably stay away from Atom CPUs (Celeron or i3 is fine) and any exotic hardware, use whatever ITX case you want and a PicoPSU, stuff it with a 2-4 port NIC and you're good to go.
