What makes you think so? Of course, it is how we understand 'decent'... But I agree that Gilgamesh is a bit too 'obscure'.
http://en.wikipedia.org/wiki/Sumerian_king_list
Queen Kubaba ,
Ur-Zababa,
Lugalzagesi or
Sargon the Great for example are great choices in my opinion.
Sargon never was the leader of the sumerian civilization, but the akkadian one (which Babylon and Assyria are both). Although the territory was the same (enlarged by Sargon and his descendants until up to the dynasty demise), sumerians and akkadians were ethnically different, akkadians being semitics (like arabs, hebrews, ...), while sumerians ... where that's what is th most interesting with sumerians : they don't belong to any ethny we know ! Besides, Lugalzagesi has the problem of being to short lived, and his conquests were completely shadowed by Sargon of Akkad (who, by the way, made Lugalzagesi walk on a donkey in front of his troops, after his loss). Instead of going with people like Gilgamesh, I'd go with somelike Ur-Nammu or his son Shulgi, who really made a unified sumerian empire, and who really existed and made a long lived royal career.
Here goes my list :
Sumeria, led by Ur-Nammu (It's just the most ancient civilization we know, in the modern sense of the term)
Israel, led by Solomon (At least, judaism would get a civilization with that religion, and second, their influence is high, like or not)
Kongo, led by Nzinga (I'd love another sub-saharian civilization, who actually was long-lived kingdom)
Yugoslavia/Serbia, led by Tito (I know some say just another european civ, but this one not only fits with the ideology theme, but also was the leader of the non-aligned during cold war. Like it or not, under Tito, Serbia/Yugoslavia was highly important)
Tibet, led by Songtsen Gampo (I don't actually care if China doesn't like it. We're not even talking about the Tibet today, but an empire which existed more than a millenia ago. Tibet higly influenced Asia during that time, and I'd love to see them in the series)
Khmer, led by Jayavarman VII (Well, maybe because they had what certainly was, according to historains and archeologists, the greatest capital of the world during their time. Because of their genious engineering abilities. Because of their long-lived empire. Seriously, I'm sure that saying Khmer is the same than Indonesia or Thailand, it's like saying that Germany and England are the same ! Yes sir, I'll go that far, eurocentric villains
!)
Last but not the least, Timurid, led by Tamerlane (The third empire of the steppes. Certainly another warmonger, but a warmonger with artistical tendencies ! Seriously, the timurids were highly proficient during their time, made great impression in the west, the european court all trying to gain the friendship of Tamerlane out of fear. He also destroyed, razed, killed huge amounts of cities and people (17 millions during his live !)