Sumeria should be no-brainer if you ask me. The very cradle of CIVILIZATION. They gave us writing, cities, agriculture, justice system, surgery, metalsmithing, pottery, domestication, organized religion, recording of history and time, you name it, they gave it. As for leader, it would have to be Gilgamesh I guess.
China is in of course. Leader Qin. They were living in palaces when we over in western europe were living in dirt huts. Always been a major influence in Asia and the world.
Same goes for India. Unsure about leader, though.
Mongols in. Anyone capable of of conquering most of asia and parts of europe deserves a slot IMO. Djengis Khan?
Japan - unsure. Where they really such an influence before the modern age?
Scandinavia - Yes, the "bloodthirsty" and "barbaric" vikings where great explorers, important traders and settlers. They established many russian principicalities. Where important and highly regarded mercenaries for the byzantine emperors. Settled parts of britain and normandy and "discovered" america. Ragnar is fine.
Germans - of course. They have always been a LARGE influence in european history. Not just counting the WW's. Bismarck is good.
France - naturally. A world-spanning influence grants them this slot. Charlemagne would be better I think
Spain - Same as above. Leader, not sure.
England - Same as above. Liz is good.
Rome - of course. Caesar is good.
Greece - Passed on and created the foundations of western society. Alexander is good. Come on, he IS the GREAT.
Russia - not a unified russia till late, but a significant world influence. Peter the Great.
USA - basically same as above. Lincoln is good.
Egypt - The most ancient "civ" still going. Not Cleopatra. Ramses the 2. is fine I guess.
Arabs - significant world religion and culture. Not sure about leader. Maybe Abu is good enough?
Incas - more a civ than aztecs if you ask me. Aztecs never were that many for that long. They made great feats of construction and infrastructure. Can't remember any leader names.
Persia - maintained world power-status for a looong time. Xerxes is good.
Byzantine - Roman heirs. Held back arabs for 500 years. Scientific, religious and industrously important. Constantin perhaps?
Portugal - World wide influence and power in history. Not sure about leader.
Dutch - Same as above
Koreans - Not sure. Don't know enough of their history I guess
Khmer - Same as above.
Zulu - Not sure. Would like to have at least one sub-saharan civ, though.
Babylon - They can stay. They keep the sumerian legacy going and historically and biblically important IMO. Hammurabi is fine.
Hebrew - yes, maybe not that great as a nation(?), but very important historically/religiously. Solomon.
Aztecs/Maya - Not sure. Would like one of them, though. Don't know that much about mayans.
Iroqouis - Big questionmark, should they be in?