While there’s plenty of options out there, I’m very seriously looking at this new board. It’s an Intel based computer, seems to have the same GPIO connector as the RPi, and is probably the same size as the Pi. This seems like a good combination - going doing the Intel route avoids (hopefully!) some issues with ARM computers with serial ports etc, it has on-board eMMC, and having a common GPIO connector is good for expansion…
It’s more expensive than a Pi, but a lot less than an NUC.
(just thought I’d throw it into the mix 
).