If you're just talking about x86 assemblers, the only one I've used much is debug.exe, and I know there are better programs for writing assembly code than that. I've toyed with others, but not enough to give advice.
However, if you're interested in learning about the architecture of the lesser-known mainframe, I recommend a program called PC 370. It's System370 emulator for a PC, and it let's you assemble 370 Assembler and run it on a PC - the book that comes with it is quite interesting.
I'll see if I can find a link for you.
