Except...
I'm going to have to harp on about the distinction I hinted at in my last post between "General" and "President".
Both are leaders.
But there are some radical distinctions between the two.
A general leads in battle. He* has his superiors, but he commands from an authoritative position. A president leads from an office. His authority is below no one's, but he cannot command in the same unilateral manner--in other words, he must be a politician.
Gandalf, I reiterate, does not strike me as a president. His leadership is very much, when he assumes it, unilateral and focused on a particular goal (a military campaign, even). He does not have a long-term authoritative mandate.
And as for "Steward of Middle-earth"... presented as such this is almost uncanonical, and even though Gandalf tells Denethor that he is a steward too, I think it's very clear that he means it in the sense of a Caretaker, which is his entire point, because there's a huge difference between the role of a caretaker and the role of a "president", which is precisely how Denethor saw his office.
Actually... now that I mention it... Denethor might get my vote. Sorry, Saruman.
*"He" is used only because Gandalf is a "he" and for convenience. It is fully acknowledged that the offices of general or president need not be masculine only.