The ancient Sumerians lived in the region of Mesopotamia, located in modern-day Iraq.
When speaking about the "greatest" civilizations to grace the world, we must include the Sumerian civilization. I say we must for two reasons: 1) It was the first civilization (that we know of) and 2) because the Sumerian civilization left a massive impact on the religion and cultural thought of the world in a way that no other civilization has since. But first, a bit of history. The ancient Sumerians lived in the region of Mesopotamia, located in modern-day Iraq. Because of its location near the Tigris and Euphrates Rivers, the Sumerians could forgo the hunting and gathering lifestyle and begin farming, which later formed some of the Earth's first cities. One such city was known as Ur and is the birthplace of the Biblical character Abraham who is credited with founding not one but three world religions: Islam, Judaism, and Christianity.