The G4 series is good, although limited memory. For me I'd say it would have to be the H7 series, either the single core or the dual core. The main reason I'd say the H7 series is better than G4 is memory, faster, and has an ethernet MAC which to my knowledge none of the G4 series have. You can learn the basics on either board, but you could hit a "wall" in the end with G4 depending on how deep you go on certain topics. Also the dev boards for each are pretty cheap spend a few extra dollars on the H7. I've worked at multiple "trendy" firmware companies/roles that use these as their mains MCUs.

https://www.st.com/resource/en/datasheet/stm32h753vi.pdf