According to Aristotle, which element of tragedy is the most important?

The Plot is the most important part of a tragedy.

Plot

In his thoughts on Poetics (or what we would call the dramatic arts), Aristotle said that every tragedy (a dramatic, serious play) contains six elements: "Plot, Character, Diction, Thought, Spectacle, Song." Of these, he identified plot as the most critical element of drama. Here's why, according to Aristotle (from his Poetics, as translated by S. H. Butcher):

The most important of all is the structure of the incidents. For tragedy is an imitation, not of men, but of an action and of life, and life consists in action, and its end is a mode of action, not a quality. Now character determines men's qualities, but it is by their actions that they are happy or the reverse ... Hence the incidents and the plot are the end of a tragedy; and the end is the chief thing of all ... The plot, then, is the first principle, and, as it were, the soul of a tragedy ... Tragedy is the imitation of an action, and of the agents mainly with a view to the action.

You'll notice in the quoted section that Aristotle speaks of the "end" or purpose of drama as well as the "first principle" of drama, putting plot or actions into that position. This is consistent with Aristotle's general pattern of thought, which was teleological - - looking for the ends or purposes of things, and seeing those ends or purposes as the first principles of things.