Best...
... science: Babylon + Korea
... culture: Brazil + France
... diplomatic: Greece + Sweden
... domination: Japan + Zulu (continents/pangaea), England + Ottomans (islands)
Fun combinations:
Brazil + Persia (Golden Eon), Germany + Zulu (largest military ever), Denmark + Songhai (It Came From the Water), Ethiopia + Shoshone (impenetrable), Austria + Mongolia (what's a city-state?)