Like I said the; Royal Military museum in Brussels .
It all depends of what you want to see!
There is a very nice Belgian police museum in Antwerp, but you need a appointment to get to see it.
We have the hat of Napoleon Bonaparte in display at a other museum in Antwerp for the momment.
For a very nice and complete colletion of German spike helmets in a private museum, you should go to
Fort de la Pompelle - Wikipedia, the free encyclopedia
Bastogne and La Glaize if intrested in the battle of the Bulge.
Ieper for the
In Flanders Fields Museum - Wikipedia
But we have also a playing card museum, old tram's,there is a great exibition about the Etruscan civilization etc...
Take a look here;
List of museums in Belgium - Wikipedia, the free encyclopedia
If you need more info about a specific museum feel free to ask.
Always looking for Belgian Congo stuff!