I am aware of this question, however that is primarily about which passport to present to whom within the airport.
I have two EU nationalities: Belgian and Finnish (born Finnish, acquired Belgian) Now, obviously, when entering specifically either of those two countries, I use that country's passport. But what about other EU countries? Do I base it on geographical proximity (eg use Belgium to enter France or the Netherlands and Finland to enter Estonia - less clear in other cases)? Whatever my primary country of residence is? Language? Vague notions of country friendliness?