If LTM is enough, positively the brightest would be a M (or even brighter a Zeiss Ikon) with thread mount adapter. Next to that, the Bessa R.
If it must be a Barnack Leica, the IIIg is the only one with a fundamentally redesigned finder - finder differences on the previous Barnacks are small enough that condition will usually be more relevant than type or age. Some of the classic barnack style Japanese LTM cameras supposedly have brightline finders, but US or Japanese contributors will know more about that - Canons or Niccas are as rare as hen's teeth here in Europe.