D1 Mini breakout
They're cheap, the ESP8266 chip is powerful and can be used with several development environments/languages. You can program them in C/C++ with the official Espressif SDK, but also the Arduino IDE where support is excellent. They'll also run more modern interpreted languages like MicroPython and Lua, again with good support if some memory limitations.
What they do lack is such a large ecosystem of 'shields' and breakouts. Wemos make their own but they're all a bit 'Arduino sensors 101' type things.
I saw these chunky screw terminal breakout boards mentioned on Twitter and while I have no immediate use for them I picked a couple up because I know they'll be great for prototyping stuff.