Yeah, if I really took this question seriously, I would say JavaScript. It runs everywhere, it does everything, and it's honestly pretty fun to work with, when it's not being awful.
Espruino is a JavaScript interpreter for microcontrollers. It is designed for devices with as little as 128kB Flash and 8kB RAM.
Before anyone says that an ESP32 isn't actually a microcontroller let me just point out that it doesn't really matter. A few years ago a board with an ATmega 2560, lets say an Arduino, would cost around 30€, today you can get ESP32 boards with WiFi and way better performance for 3€ making the Arduino 2560 mostly useless / not cost effective.
In just 5 years with all those new RISC and ARM chips will most like triple their performance and take over what is now done by high end AVR and PIC microcontrollers. Costs will go even further into cents and microcontrollers as we know them will become irrelevant. With the extra performance JS will become the most popular language simply because developer time is more expensive than hardware.
I used a few for high speed logging of a quadrature rotary encoder to measure the speed of a hydraulic sampling arm. Battery powered , had a 4 line LCD and keypad with a simple menu interface to take samples, as well as a serial export function to get .CSV files via hyperterminal.