I've always been of the opinion that "interesting" leaders are a better choice than historically accurate or even great leaders.
For that reason, I am more forgiving about females who weren't actually leaders of their country (Joan of Arc) or whose accomplishments were dwarfed by their disastrous policies (Cleopatra). It breaks up the sausage fest.
I also think name recognition is important, which might make one leader preferable over one who was less well known or had a name that was particularly unpronounceable.
As for the civilizations themselves, I think they have to be weighed along similar criteria. Sure, the Zulus had a short run and limited greatness, but they're well-known and their badassitude is undeniable.
Because every game of civ takes place in a fictional world, there's no need for strict historical accuracy, and I would personally prefer that the game mechanics fit more of a historical model than the names and places... but everyone has their own criteria, which is why I think the game developers have often preferred name recognition, gender and ethnic diversity over all other criteria.