A pax CAN be rude regardless if they are white, asian, black, you name it.... nationality doesn't really matter, let's not start discriminating.
Quite so - the problem with this thread is that invites us to group people we find rude by nationality, race, religion or even the purpose of the trip. In doing so, we appear both absurd and bigoted.
The truth is that the "rudest pax" are in fact those individual human beings that are rude, discourteous or insensitive. A pre-conceived idea of what your passengers on any given flight/route will be like simply ensures that those ideas will be re-enforced, one way or another. We all remember the difficult people, and tend to forget the overwhelming majority who are normal, pleasant and friendly human beings.