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...