Prime Minister of Luxembourg
Since the foundation of the party system, only one cabinet (between 1921 and 1925) has not included members of more than one party.  Most of the time, governments are grand coalitions of the two largest parties, no matter their ideology; this has made Luxembourg one of the most stable democracies in the world.  Two cabinets (between 1945 and 1947) included members of every party represented in the Chamber of Deputies.