For the EU, the country where you will spend the most days is the country where you must file your application for a shengen visa.
The visa process will be easier if you have one country for arrival and departure. For example, you can plan to enter via Frankfurt then go around Belgium and the Netherlands and then leave via Berlin. But for purposes of the visa application, you mist pretend that you’re staying only in Germany. All you need is to show that you have hotel bookings in Germany for the entire duration of your trip. You can cancel the unnecessary bookings later.