Fitlet2 looks rather nice to me. Outfitted with an Intel J3455 CPU, and 2-4 Intel NICs, it is really power efficient for its performance class (idles at ~6 watts, for those that care). There are also some Chinese companies producing slightly cheaper boxes in this category- Qotom, Kettop, Protectli.
When it comes to software, I'm conflicted. I like pfsense, but Netgate has gone a bit sour with the FLOSS community. I'd also consider OpenWRT, FreeBSD, OpenBSD.