My strategy is to start in the North and blitz along the coast.
1. Position tanks and motorized in the northernmost province of the Dutch border. Declare war only on the Netherlands, attack along the entire front with conventional forces and use light armour and motorized to head west and take Amsterdam and the entire coastline. It's a very quick campaign.
2. Place tanks and infantry on the northern Belgian border, tanks and mot. nearest the coast. Declare war on Belgium, blitz down the coast with LT and motorized into France. Head for Paris, then from Paris for Brest. It works every time for me.
Speed is of the essence here, hence LT and mot. The Belgians don't know what hit them, and the French are wholly taken by surprise, and your light tanks and mot. can quickly outrun their infantry. I never attack Belgium in the west or south-west, it turns into a slugfest and they get reinforced quite quickly by France.