STM32s have great Rust support[0] if that matters to you. Personally I strongly prefer the embedded_hal[1] and RTFM[2] APIs over the Arduino ones, although the peripheral coverage is a fair bit weaker. Their DISCOVERY kits[3] (preassembled boards that are ready to program, just like Arduinos) are ~$20, and also double as programmers if you eventually end up designing your own boards.

[0]: https://docs.rust-embedded.org/discovery/index.html

[1]: https://github.com/rust-embedded/embedded-hal

[2]: https://github.com/japaric/cortex-m-rtfm

[3]: Such as https://www.st.com/en/evaluation-tools/stm32f3discovery.html

I love the TI launchpads.

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

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.)

ESP8266 or the ESP32 (if you want something beefier that also has bluetooth) give fantastic bang for your buck, especially if you get them from a trustworthy-ish store on Aliexpress

The ESP8266 is a fantastic platform and Adafruit has some great breakout boards for it. I’ve heard good things about its successor the ESP32 as well.