I\'m unable to highlight my assembly.
GNU Assembler (GAS) x86(-64) syntax:
I know there's a chosen answer, but for anyone else who comes here in the future, you have to specify which syntax/machine you want. Assembly comes in a lot of different flavors for a lot of different machines. Other answers have given Intel x86, NASM, SPARC, and MIPS. When I visited this page back in 2014, I was looking for a good GAS/AT&T syntax def for x86, which I never found.
So naturally, I made my own. It's really an x64 highlighter, but of course that includes x86 too.
It's also on Package Control. To find it, type "gasx" into the search field.
I also ported it over to Atom (available from Atom packages), Vim, and Gedit.