Final answer:
The rivers/water had the greatest impact on Mesopotamia, providing the means for irrigation which was essential for agriculture and the emergence of civilization in the region.
Step-by-step explanation:
The greatest impact on Mesopotamia was made by its rivers/water. Mesopotamia, also known as "the land between the rivers", owed much of its success to the Tigris and Euphrates Rivers which provided a constant water supply essential for agriculture. The flat land and temperate climate were also conducive to agricultural development, while the fertile soil supported the growth of crops such as barley, sesame seeds, and dates. The control of water via irrigation and the ability to grow excess food were central to the rise of famous cities like Ur, Babylon, and Nineveh, and to the region's emergence as the Cradle of Civilization. Without the rivers and the resultant irrigation systems, the easy to manage flat lands, favorable climate, and fertile soil would not have been as effectively utilized, making the control and use of water the most impactful resource for Mesopotamia's development.