I have been using the iconic Arduino Uno R3 for classroom projects for many years now. The boards are easy-to-use, reliable, and inexpensive (important, as eagerly enthusiastic hands playing with power supplies and soldering irons have been known to occasionally fry them).
After many years, the good folks Arduino have updated this classic workhorse, and the new R4 version is a thing of beauty. It now features a faster 32-bit processor, more memory, onboard Wi-Fi and Bluetooth LE, an RTC chip, USB-C, backwards compatibility with same form-factor, and (somehow, they kept this less goodie a surprise) an on-board 12×8 LED matrix (perfect for displaying real-time application feedback).
I’m already redoing some of my lessons, starting with using the LED matrix as a way to visualize arrays (and especially multi-dimensional array), a topic that can feel a bit abstract and esoteric to new coders.
Arduino Uno R4 is a game changer!