Cross-Platform Assembly Language Compilers?

[cyaiplexys]

Thanks to a very nostalgic thread, now I'm curious about something else. 
Back in the day I used to do assembly language programming on a Tandy 
Color Computer 3 with Disk-based EDASM+.

I'm wondering what is the best assembler/compiler for the x86 (64-bit) 
system?

I'd like to try a native compiler but also I would like to have 
something I could compile for Arduino (here we go again) and ARM and 
other CPUs as well.

However, for now I'd like to just stick to x86. I would imagine that 
would be easiest since I could test right on the laptop. And it's 
something I've always wanted to learn.

I don't plan to really do anything major with it (we have C, Python, 
Perl and a host of other languages for real programs).