Honestly I still haven't had a chance to try them out myself so I can't make a specific recommendation but that market has been exploding recently. I have a sort of nice problem where people keep gifting me their Raspberry Pi's once they aren't sure what to do with them so I keep accumulating them without trying.

That being said, the big ones I've had my eye on lately are things like the Odroid N2+, the Jetson Nano, the Rock Pi or the Banana Pi. Some of these cater more towards being integrated into projects that need a lot of GPIO, others are focused on just being a low cost low power headless server or thin client.

The SBC market seems healthy enough that by the time I need another SBC I'll have a lot of options. Biggest loss is just that having one extremely popular hobbyist board made it really easy to find solutions to issues in the community and now there is just a lot more variety out there.