What is the most popular system of government throughout the world

The Roman conquest of Italy was the result of a series of conflicts in which the city-state of Rome grew from being the dominant state in Latium to become the ruler of all of Italy. The first major Roman conquest in historical times came with the final defeat of her neighbour Veii in 396 BC. In the second half of the 4th century BC Rome clashed repeatedly with the Samnites, a powerful tribal coalition. By the end of these wars Rome had become the most powerful state in Italy. The last threat to Roman hegemony came when Tarentum enlisted the aid of Pyrrhus of Epirus during the Pyrrhic War (282 - 273 BC). By 218 BC Roman conquest of Italy had been completed. Conquered territories were incorporated into the growing Roman state in a number of ways: land confiscations, establishment of coloniae, granting of full or partial Roman citizenship and military alliances with nominally independent states. The successful conquest of Italy gave Rome access to a manpower pool unrivalled by any contemporary state and led the way to the eventual Roman domination of the entire Mediterranean world.

I would probaly have to say democratic. 

River valleys were good places for hunting and fishing.