the ESP’s (NodeMCU’s) are already one I play with, they are cheap and easy to use, and have wifi (kindof). STM32 (Blue Pill) sounds promising, but I haven’t had a chance to try one out yet.
MSP430, I tried these out but the dev tools for them stink.
https://www.makeuseof.com/tag/best-arduino-alternative-microcontrollers/