[Doria]

"If Iskander was the only one to figure out Gilgamesh's true identity without being told explicitly, would that make him the smartest Servant or was he just more knowledgeable in that respect?"

Yeah. As stated above, he was the only one who really approached things like a (somewhat haphazard) military campaign. Additionally, dying in Babylonia may have made a slight impression on him, in life...

Then again, Gil figured out who Berserker was (I think that this was not as explicit in the anime, but was clearly stated in the book), so he was obviously able to discern stuff when he set his mind to it. He also had the benefit of the early Assassin intel, but I don't really know as that helped him very much.