I think early game, Monarchy is the way to go, but as the game progresses, it will slowly all turn into Constitutional Monarchies and Republics.
Out of the starting nations, I believe they are the following:
France - Monarchy
Sweden - Monarchy
Ottoman Empire - Monarchy
Great Britain - Constitutional Monarchy
United Provinces - Republic
Russia - Monarchy
Spain - Monarchy
Prussia - Monarchy
Poland-Lithuania - Republic
Austria - Monarchy
Marathas - Monarchy
I am not entirely sure if some of those monarchies are constitutional, I am just working from my base knowledge.
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