If you mean by important the STARTERS of civilization, the answer can be easily attained by looking at who were the first sedentary civilizations that first started the various traditions and cultural constants in various areas on the globe.
-In the Far East (China) it started with the Yellow River civilization; the Xia Dynasty c. 2205 BC
-In India/Pakistan: The Indus River Valley civilization; 2600-1900 BC
-In the West: Mesopotamia (Sumerian proto-literate period: 3000-2500 BC) at the Tigris and Euphrates, and Egypt (circa 3000 BC) along the Nile - Greek and Roman culture never would have arisen if not for these two civilizations.
-The Americas: The Olmecs around the Coatzacoalcos river basin; circa 1400 BC.
It is widely agreed upon by historians that one of the most important developments a society can make is that of irrigation farming since these yields a regular output of food surpluses which allows a civilization to consistently grow; hence why all the above mentioned sprang up around fresh water rivers.
