The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Enmerkar was considered the greatest Uruk leader because he was a great hero that dominated his rivals, including their enemies in Aratta.
His prowesses are written in two Sumerian epics titled "Enmerkar and the Lord of Aratta," and the second one is "Enmerkar and Ensuhkeshdanna."
Along with Gilgamesh and Lugalbanda, Enmerkar is one of the most prominent figures in the stories of Urk, in Mesopotamian times, the oldest civilization on planet Earth.
The most famous epic of those years is probably The Epic of Gilgamesh. We are referring to the oldest piece of literature known by man. The Epic of Gilgamesh tells the story of King of Uruk, Gilgamesh, and his adventures. This story dates back to the times of the oldest civilization on Eart: the Sumerians. These people settled in between the Tigris and Euphrates Rivers, in the Middle East region, of what today is Iraq.