To me, comparing Civs and leaders is usually like comparing apples and oranges. Different times, and different circumstances lead to very different events.
Gilgamesh
Some believe Gilgamesh is only an epic warrior taken out of Sumerian legends, however, since his name appears as one of the Kings of Uruk, most scholars believe he was indeed a historical figure. The legends are likely a direct or indirect result of Gilgamesh's propaganda, so there's no real way of defining what accomplishment is fact, and what is just a tall tale.
Personally, I have no qualms about a half-mythical leader, especially for a civilization for which recorded history is basically a list of Kings and some recorded conquests.
With only bits and pieces of Celtic recorded history, it's also difficult to find an optimal leader for the Celts. Since Celtic history was an oral tradition that has almost entirely been lost by now, all we really know of their history is mostly collected from other civilizations that have interacted with them. And the latter would mostly stress the times they won against the Celts, and not the times they lost. So the Roman victory against Vercingetorix is well known, while Celtic wins are mostly forgotten. As one of the few Celtic leaders billed with a win,
Brennus becomes a decent choice.
Boudica is important because of the legendary status she gained post-death, but is certainly questionable in terms of accomplishments. But to me, fame and legendary status can be an acceptable criteria for a civ leader, especially when fairly little is known about a civilization as a whole.
The one truly bad choice, in my modest opinion, is
Montezuma, also known as Moctezuma II. Why oh why was he picked over Moctezuma I (who's real name was possibly Ilhuicamina) is beyond me.
Why chose Montezuma, who inherited an Empire at its height and essentially dropped the ball when it came time to defend it over the ruler who expanded it the most is simply mind boggling. What the heck were the Firaxis guys even thinking? That's kinda like choosing Richard Nixon as the premiere American leader!
