AVR assembly is relatively easy to learn, especially if you're interested in microcontroller stuff: you know, timing and a lot of hardware interrupts. for something more practical, ARM64 isn't bad either and it will introduce you to RISC pipeline nicely although it tends to have multiple similar instructions for slightly different things despite being RISC. 32-bit ARM is much messier though given the posssibility (and occasionally - depending on restrictions - necessity) of jumping between 32 and 16 bit modes.
I do not recommend learning x86 assembly because it's very complex due to neverending feature creep and different vendor-specific extensions. also, there's literally no reason to learn it unless you're interested in low-level hacking/exploitation/vuln research (which I personally am, although I'm currently doing it with ARM64) because compilers are generally better at optimizing for it than programmers are and unlike alsmot every other fucking assembler it doesn't even make it easy to write constant-time code. it doesn't even guarantee that things you put there will be executed at all (
http://blog.erratasec.com/2015/03/x86-is-high-level-language.html)