You should be flooded with replies on this one. Just thinking of three cities I know, they all have their fall guys for the jokes. In Cologne, you can always get a laugh at the expense of people from neighbouring Bergheim, which is to the West. In Basel, they make jokes about people from "Klein Basel" - on the other bank of the Rhine. In Linz, Austria, they get a lot of mileage out of jokes about people from the "Mühlviertel", whom they also call "Urfahrmarkter", meaning folks to the North of the Donau.
It will be interesting to hear what various Scots, Irish, Americans, Canadians, Aussies and New Zealanders can add to this list!