I have a few, but voted for Britain.

My selection would be nations able to project their power to other parts of the globein order to enforce their will.
Mongolians-highly mobile and feared instument of war.

Romans-I belive they did this on many occasions, and is what caused them to annex so many areas.

England/Great Britain-Ruling over the largest Empire the world has ever known, and the submission of many nations through its powerful navy and thus its highly proffesional army.

France under the Bourbons(until the seven years war)-The most powerfull nation in the world for around a hundred years. Although its bid for European Hegemony and consequently global hegomony were destroyed in the seven years war, it was the deciding factor in the American Rebellion.

U.S.A-No comment is really needed.

USSR-Niether is one here.

I am tempted to put Nazi Germany in.