Le Refuge des Fondus was recommended to us as the "best fondue restaurant in Paris" and (in my limited experience) it seems to live up to expectations. 

There are two long tables formed by pushing many small tables together with no spaces between them.  This makes it moderately difficult to get to a seat along the wall, but it gets everyone into a party mood if nothing else.  Patrons wind up sitting shoulder to shoulder with other people they've never seen before and may never see again.  We sat between a couple out on a date and a group from the Netherlands who had sailed their boat to Paris and docked it along the Seine.