Eisenhower, managed to keep the various big ego personalities in the Allies from fighting each other (Montgomery, Patton, deGaulle) and without being an overbearing tyrant. Leading people is difficult, but leading men who are used to being in charge has to be an order of magnitude worse.
For me, it has to be George Washington. Cliche, perhaps, even when he was still alive he was called the father of his country. Though not the best general of the time, he held an army of independent minded colonists together against the superpower of the age. He refused to be named King after the war, and refused to stand for a third term as President. He pretty much defined the office of the Presidency, setting standards of behavior for generations of future leaders.
Read about the Newburgh Conspiracy, a possible uprising in the Continental Army over months without pay towards the end of the war. I won't bother pasting in the entire wikipedia entry, but Washington spoke to an assembly of officers and essentially shut the whole thing down. A memorable quote from this speech, where he had to put on glasses to read a letter from a member of Congress- "Gentlemen, you will permit me to put on my spectacles, for I have not only grown gray but almost blind in the service of my country." Many of his officers were moved to tears at this, and I'm not ashamed to say I was as well.
Probably too obvious for a school thing, though.
Edit- a different source than wiki says Washington's speech was not very well received, his men were still angry and still wanting to challenge Congress. It was apparently only the spectacles comment that reminded everyone of the very human nature of the man, and that he had suffered alongside them. That they had this much respect and even love for him seems like a sign of a great leader.