GermanyοΌthe third reichοΌin HWW.
The country have the mightest troop in central Europe, which allows u to take Poland in 5 hours, France in the next 8 hours. Which really means u can conquer the most land in Europe in half a day.
Germany has the best research progress in HWW game mode, so u can start in a better place, but when ruling the country, should be careful to be attack in all sides. which means u must have at least 1-2 allies.
In my games, I am only active when playing as The Deustches Reich (The German Empire)(or sometimes italy and spain).
I have never lost when playing the country, no matter in HWW or Blitzkrieg.
So I strongly suggest Germany.
And btw, its hard to get the chance to play as the country in HWW. The Next choice is Spain or Italy. The first have quite a few troops and many good industries.Spain should focus on southern France, and Northern Africa( France). But Spain must ally up with Germany, cuz it can easily eat u becuz of its strong force. Italy empire is actually not very strong, which is challenging and fun. Though the Italian Navy have quite a few battleships of lv.2, but in destroyers and submarines, its very weak(espeacially against France and Britain.) Italy should aim at the whole Africa and the Balkans.
My suggestion is first take British Eygpt, and go take France-northern Africa. Company with Germany in Europe, and attack British land in Africa, (better not company with South Africa, it may grow too big and become a threat.)
I can't give u a full plan of the game, cuz u need to take new actions since new problems broke out( just as Moltke said.)
But I can give you one for the first 5-18days. Good luck.