General question on choosing an Assembly language based on my goal
There is very, very little need for assembler. For is very rarely need for inline assembly - for example, I have some code somewhere that calculates (a * b) / c for unsigned 64 bit integers where a * b overflows. 

And asking "which assembler should I learn" - if you ever need assembler, then that's the one you need to learn. Any other assembler is useless. 

The only case where you really need to use assembler is at the extreme ends: When you are creating a virtual machine, you need to understand the extremely rarely used instructions to do this safely. If you create codecs, or encryption software, where you have very small amounts of code using up huge amounts of CPU time, that's where you may need assembler. But you don't need to know just assembler. You need to know and understand CPU instructions that have been created exactly for these purposes. You are in a highly specialised area there.