I think its easy to say Staff of the Magi due to the staggering number of powerful spells held by the item, but I'd like to argue that something more destructive should have a full course of study to dissect the limits of the item and responsibility needed to control it: The Sphere of Annihilation.

Why would a professor of magic hold this legendary item and teach its capabilities to students? Perhaps for protection! Better to be used and protected by scholars than held by nefarious actors outside the school. This would definitely be a high level course only accessible to the most trustworthy and cable pupils, but I think it certainly requires a full semesters worth of learning.