My personal preference would be: 100000 (monetary symbol), but, I also understand that others might prefer the symbol before the numeric (i.e. $100000), even though you would not say "Dollars one hundred thousand" in English.
The closest you get to an international standard (that not many use) is the International Bureau of Weights and Measures recommending (ideally thin) spaces (so "10 000")
Interesting! Do you know of any countries that use the underscore? I believe I've seen France(...?) use spaces as a separator, but I've never come across the underscore.
Personally I would like some means of breaking up units of triplets that could be used universally. It's frustrating that there isn't a standard across the globe for such crucial means of expression. It wasn't until fairly recently that I learned that the comma wasn't the universal standard.
I am not aware of any country using the underscore to separate multiple of thousands, I just thought of it because it can be used for that purpose in some programming languages. Ans if it's true that no country is using it, then I guess it could be used as a 'neutral' way of doing it.
Spaces cause ambiguity with separate values like 123 456. Underscores are ambiguous with missing values due to the common use of a line segment to indicate where to fill out form values in addition to the same separate values issue as spaces.