Hi Bill,
Well, I don't know if it's the best (which is probably a matter of opinion
), but I personally prefer Pollyscale Flat Clear. I thin it with a little distilled water, although I've heard others say they don't thin it at all. I get a very flat finish, which is what I'm usually looking for.
I haven't used Tamiya or Gunze flat, so can't comment on those.
Mark