I'll try and stick with Civs that have been in before (so no Ashurbanipal II for Assyria):
Ancient:
Rome - Marcus Aurelius, Constantine, Scipio Africanus, or Marius (tried to pick a good variety of Republic and Imperial figures)
Babylon - Nebuchadnezzar (while the Chaldeans aren't the Babylonians we'd normally think of for classical Babylonia, he's certainly a famous historical figure).
Celts - Vercingetorix (I'd probably stick with Brennus, but, if we had to pick somebody new, he'd be a good choice).
Western:
America - Thomas Jefferson or Theodore Roosevelt (I think if the President appeared on Mount Rushmore, he would be a good choice).
Spain - King Charles I (Charles V, Holy Roman Emperor. Equally German, but the new world empire he owned is associated with Spain).
Scandinavia (yeah, Vikings in some games, but I'm trying to broaden them from their pagan roots) - Gustavus Adolfus (very important King for Sweden, associated with the 30 Years War)
Ottomans/Turks - Ataturk (similar to the Scandinavians, it depends on how broadly you classify them).
Eastern:
Japan - Meiji (the Meiji restoration was an important time for Japan and gives them more representation than feudal Japan)
Korea - Sejong (another important Korean leader, probably more important than Wang Kong)