Most Important Presidential Actions

On President's Day, let's consider what Presidents have and have not done. What were the most historically important actions or words, both positive and negative, that have come from American Presidents?

I'd put Washington's stepping down after two terms as the most formative action by a President. The signing of the Emancipation Proclamation by Lincoln as the most morally necessary act by a President. And Eisenhower's Farewell Address as the most insightful speech by a President.

Others?