Technology or Peace

[Sir Beane]

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.

Poland-Lithuania is actually a Constitutional Monarchy at the time the game starts, all the otehrs are correct .

You can check on a recent IGN screenshot which shows the diplomacy screen for the playable factions.