6502
The 6502 processor has a special place in the childhood memories of many people, because it was at the hearth of many micro (and not so micro) computers of the 80s. The Apple II is probably the most iconic, but for many of us the real winners were Vic-20 and C64. “The” 8-bit home computers, together with ZX Spectrum.
I find 6502 Assembly a good start for who wants to play with bits, bytes and registers. It’s easy enough not to be intimidating, but it’s a real Assembly with real potential applications. And there’s plenty of online courses to learn it, even if most of them are focused on retrogaming platforms like Atari VCS and Nintendo NES..
It’s also a good way to learn “programming with limitations”, since systems like Atari VCS didn’t offer that much in terms of digital resources (even the 6507 CPU wasn’t at the same level of the official 6502). That said, there’s plenty of room for improving your low-level programming skills. And then jump to more complex platforms.
Also, even if it can sound a bit esoteric, there are different ways to emulate a 6502 processor using simple SBC boards. This is kind of obvious, if you think that a modern board controller is more powerful than a 6502 chip. So, you can have your little portable retro machine, customized for your own digital tastes and ready to play your own games.