Who is the highest-ranking soldier in your country?

This is inclusive of the country's entire history, not just world war 2. For some, it is explicitly given, like in the case of the US. There were three men awarded six stars by act of congress; George Washington, John Pershing, and George Dewey. It was further provided that Washington will be regarded as the senior general of the US armed forces and no American soldier could ever outrank him.

Other explicit promotions (albeit self-bestowed) I know are Muhamar Kadaffi (colonel,) Kim Il Sung, and Kim Jong Il (seven stars each,) and Idi Amin (field marshal.)

Others appear to be honorifics and not real promotions above field marshal. You have Herman Goering as marshal of the reich, and a number of soviet generals who became marshal of the soviet union.

What about other countries? Were there field marshals in Canada and Australia?

 

France oldest Maréchal stems from 1185, so it would hard to beat over 800 years of history . They score 342 . The most famous ones accross the Atlantic are the Louis XVI ones with Rochambeau, Choiseuil and Noailles because they are part of the U.S. Indenpence war . Then of course the Napoleonic ones are world famous with names as Davout, Kellermann, Poniatowkski, Grouchy, Ney, Lannes, Murat etc... Then the WW1 ones Foch, Pétain , Joffre and the WW2 ones : de Lattre, Juin, Leclerc and koenig.

Many other famous names rings bells too (Crevecoeur , Montmontrency , Colligny etc...)

Among their ranks one can note some prestigious foreign names with Scottish (Stuart) , Polish (Poniatowski), German (Hohnelowe) , Flemish (Borselleen) , English (Fitz James) origins.

https://fr.wikipedia.org/wiki/Mar%C3%A9chal_de_France