The most important speech the president gives to Congress is

The most important is the State of the Union Address.

Explanation:

The State of the Union address is the speech that the president gives to the Congress every year. This is an annual (yearly) message or speech in which the President talks about the country's budget and country's economic report in the starting of the calendar year.

Apart from this, the President also comes up with a legislative agenda and national priorities. Through this report, the Union gets an idea of the country's past situation and on what areas they need to focus more.