Well, this kinda violates the terms of this newsgroup (no flames),
but I'll post anyway. Here's the good thing about four major assemblers:
TASM - I think this is technically the best, but it's hard to find, and if
you don't buy directly from Borland, it's expensive. I also hear that it
has poor documentation. I think it's fully compatible with MASM, or at
least up to some older version (3.0?)
MASM - I think this one is slightly inferior to TASM, but I've heard the
documentation is better than TASM's. It's also expensive, even if you buy
it directly from MS.
A86 - The best thing that this one has going for it is that it's cheap (I
think $50 is the registration price). It also has a shareware version. I
don't know much about this one since the last version of it I had could
only do 286 operations, but I'm fairly sure this has improved.
WASM - Watcom's assembler comes with Watcom's C++ package, which I think
is the best (game) development package for DOS in existance. It comes
with DOS4/GW, too. I don't think you can buy it seperately. This one's
great if you want to do some DOS games, but otherwise it's basically
worthless.
Well, I tried not to be biased, but I think the best overall is TASM (but
I'm not fortunate enough to own it).