I am gonna post for each civilization including Babylon.
America - Washington was clearly the best choice. He was one of the founding fathers and also was the general who lead the American forces in the Battle of Yorktown. He was also the first President of the US. Other good choices would have been Abraham Lincoln one of the most famous president of the US which led them through the civil war and also the man who abolished slavery in the union. Woodrow Wilson I find it strange why he has never been in civ before. This man is in my top 5 for US Presidents. He lead America during WW1. He also wanted to impose less hars measures on Germany but sadly his proposals never went ahed and of course Franklin Roosevelt Perhaps the greatest US President in the 20th century. He brought the US out of the Great Depression and of the era of Isolationism. Together with Churchill he was one of the reason the Allies won WW2.
Arabia - Harun-Al-Rashid was perhaps not the best choice. But I would put it as second. He lead the golden age of the Arabian Caliphate. During which Baghdad became the cultural capital of the Muslim world. However I would have chosen Saladin but that's because I like Saladin a lot he was perhaps the greatest Muslim general of the Crusades period. He was respected by both Muslim and Christian alike and in fact the Christians recognized his brillaince as a terryfing General.
Aztec - I don't see anyone better than Montyman but that's probably I don't know much about Aztec. In my country in the History classes we talk more about the history of Portugal and then of Europe and finnally the world so the only Aztec thing we are thought about his how they were easly counquered by the Spainish. And the only Aztec emperor we talk about is Montezuma II here though I have also learned about Monty's son Cuahtemoc.
China - I belive they only choose Wu Zetian because she was a women empress. A better choice would have been Qin Shi Huang or Taizong or perhaps Kangxi.
Egypt - I agree with Ramses choice not only he was a builder but he was also a good milatry leader in his own right however the perfect choice would have been Tutomose III he is perhaps the best Pharoah of Egypt of all time.
Engalnd - I don't have a problem with Elizabeth she was a great Queen and she lead England during a difficult time when Catholics and Anglicans were at each others throats. For me though I would choose either Queen Victoria or Henry VIII if choosing a monarchical ruler or Winston Churcill if choosing a prime minister.
France - The best choice when I think of great French leaders Napolean is allways the first to come to my mind followed by Louis XIV.
Germany - Bismarck was a good choice here. He was the men most responsible for the formation of Germany he was a great leader.
Greece - 1000%behind Alexander expanded the hellinatization to almost all reaches of the known world by then.
India - Gandhi I agree as well he is synonimus with civ games.
Iroquois - I don't know much about them either him, Logan or Degnawida would be good choices.
Japan - I agree with Nobunaga though I will miss Tokugawa. I wouldn't mind Hideyoshi as well or emperor Meiji.
Ottomans - Either him or Mehmed would be the best. Though I prefer Suleiman the Magnificent.
Persia - Darius I is a good choice though I would have prefered Cyrus. When I hear about Persia I allways think of Cyrus the Great counquer of Babylon
Rome - Augustus was a great choice but I belive Julius Caeser should have been the one chosen since he has been in all civ games until this one.
Russia - Yeah defenatly Cathrine. I just wished they would have put Civrev's Cathy though
.
Siam - I also don't know much about Siam but I would have prefered the dude which the UU is named after.
Songhai - I also don't know much about Songhai to have an opinion
Babylon - Nebuchadenezaar wasn't a bad choice but I would have prefered Hammurabi.