IMHO just use whatever CPU you have available. x86-64, ARM... No need to overthink it.

Different assembly have different delights:

If you use whatever your computer can execute, it's fun to make actual, real programs for your operating system in assembly. That means likely, x86-64. (Even if you are on ARM, you computer will emulate x86-64 just fine, most likely.)

Or use ARM, if you are on a not-old Mac or whatever.

The simplest architecture which is still 32 bit and has real hardware, is probably MIPS.

If you want to feel what it's like to control a complete system from end-to-end, you could dive into embedded platforms (AVR/ARM), or emulators of classic machines like the Commodore Amiga with the m68k line of processors. It has a sweet-spot in my heart because on the one hand it's modern enough to have great graphics, sound and even network capability, on the other hand it's simple enough to completely understand.

Either way you choose, you'll have great fun.

We use ARM in our computer organization classes. It's more accessible than x86 and allows you to get a feel for the important concepts of assembly: register-to-register operations, conditional branching, and how the stack is used to manage function calls and returns.

I like the CPUlator as a platform. It lets you step through the program one instruction at a time and observe all of the registers and memory locations.

https://cpulator.01xz.net/?sys=arm

This set of lessons is a good starting point:

https://thinkingeek.com/series/arm-assembler-raspberry-pi/

My course repository has several example programs with explanatory comments:

https://github.com/dansmyers/ComputerOrganizationAndArchitec...