I would recommend taking the path of least resistance and getting something Arduino compatible. After ignoring it for years, I tried it out last week. It is very easy to use. And the M0- and M4- based boards are very capable.
The Adafruit Trinket M0 was what I needed for a recent project. It's $8 and tiny. The feather boards also look very neat, if you need BLE, the ability to stack peripherals, etc. They all integrate with Arduino so you can be up and running in a minute. (They also support Python running on the device, which is maybe even easier than Arduino.)
http://www.ti.com/design-resources/embedded-development/hard...
Seems like the number of them has gone way up so if you don't want to waste time thinking about which one to try, here are some suggestions.
Cheap and minimal ($10): http://www.ti.com/tool/msp-exp430g2et
High spec for things like motor control ($34): http://www.ti.com/tool/LAUNCHXL-F28379D