Makes me wonder what's the cheapest most efficient application class chip out there. Esp8266 ? I assume it's not the best but I'm not too aware of the MCU world.
It's always a multi dimensional spectrum of cost, performance, peripherals, development support, availability, family reach, etc. I personally really like STM8 microcontrollers for their simplicity and very low cost (can be less than 30 cents). There's actually another project that brings Forth to STM8: https://github.com/TG9541/stm8ef It has very good documentation and I recommend anyone to take a look
Today, probably the ESP32 is the most bang for the buck - unless there's something more cost competitive and equally widespread, which didn't come to my mind.
Notably, an ESP32 based on RISC-V is due shortly, with an expected price of $1. Wifi, BT5+BLE, 400KB ram (which may not be enough for everybody, but it's close), 22 GPIO's, a single 32-bit RISC-V core running at 160Mhz.