
ranked by price: stm32f103 (blue pill), stm32f103, pro micro, Raspberry Pi Pico, stm32f411, teensy 2.0
Pico is DUAL core cortex-M0+, while stm32f411 is single core cortex-M4.
Pico currently not supported by qmk, but supported by circuit python + kmk and may be zmk; all others are supported by qmk.
Most widely used: pro micro/teensy
Lowest cost: stm32f103 (blue pill)
Most feature rich/bang for the buck: pico