These are my personal favourites: https://www.pjrc.com/teensy/ https://www.pjrc.com/teensy/ I've also extensively used PIC24's and they're great (fast, cheap,

These are my personal favourites: https://www.pjrc.com/teensy/ https://www.pjrc.com/teensy/ I've also extensively used PIC24's and they're great (fast, cheap, feature-rich 16bit microcontrollers) but I don't know of any existing boards using them (I made my own) and there isn't nearly as much documentation/sample code/libraries as there is for Arduino and clones.