I'm sorry, MASM? All the cool kids use NASM or YASM.

Some still like FASM. Why NASM or YASM?

I prefer FASM but NASM is a strong second choice. But at the end of the day it really doesn't matter.. purely preference.

fasm absolutely rules on windows

The Linux kernel prefers GAS (or clang) w/ the C preprocessor. Luckily most of clang's assembler unit tests (and LLVM's MC layer unit tests) also use GAS style syntax.