I'd generally go with Wikimedians in the United States as well. Not perfect but probably the best of the options and I can't come up with something better :).
On a side note I would definitely argue that chapters (or user groups etc) are not and never have been the 'definitive group that represents' anything ;) and I know many would agree (though certainly not everyone) it's one of the reasons I've never really like Wikimedia X or geographic exclusivity. I don't think that was a good decision on our part when it was made originally but sadly that ship has mostly sailed :( .