Uno, Mega, MKR, GIGA, Portenta — and a long tail of compatible boards that share the footprint but not always the voltage. The 5V/3.3V split between the classic and MKR lines bites people more than any other gotcha here.
About 75 boards across every Arduino generation: 5V Uno/Mega/GIGA, 3.3V MKR and Portenta, plus DFRduino and Bluno compatibles. Confirm the voltage rail before stacking any shield.
