I would argue that 6502 is a bad first ISA to learn if you want learn assembly. You‘ll spend most of your time fighting the quirks of this clever, but deeply flawed architecture. The idioms you learn to work around them don‘t translate to any better designed architecture that wasn‘t constrained by the tools and budget available to MOS at the time.

If you want to learn a small, yet powerful instruction set with a few quirks go for ARM v6M. It‘s still in meaningful production (no the Monster 6502 doesn‘t count), has good platform support in the latest open source toolchains (debuggers, compilers, assemblers, linkers, etc.).

If you value openness of the architecture enough to deal with a less mature platform (as of early 2025) then pick a RISC-V MCU instead. If you can‘t decide pick a RP2350 :-).

The ARMv6M instruction set is small and no loading constants doesn‘t require long winded instruction sequences if you do it as documented (PC-relative load instead of shifting in immediate data). You don‘t have to deal with self modifying code and/or the zero page to index memory. Your registers are the same width as the address space. Yes it‘s 32 bits, but that makes it simpler to learn, use and teach than all 8/16 bit and most 16 bit instruction sets I’ve seen because you don‘t have to work around to narrow registers for common operations. To anyone who thinks this sounds boring: don‘t worry ARMv6 still has enough quirks you can use for code golfing.