I would vote for the Moose in RDR2, just because of how god damn impossible it can be to find one, but as it was not on the list, i voted for Dutch.
Not just only for his antics in RDR2, but also in RDR, he was to me an even more unique character back then, the sheer amount of characters in the gang at 1899,and he still manages to stand out.
And even if Micah is one i love to hate, he is not what i would call a permanent character, Dutch ia truly more or less the creator and devastator of the lives of many, in between the 1870s to 1911 and beyond.
Micah was part of the gang from late 1898 to 1899, then adventuring alone for about 8 years, and ended up as how we know...