Why is the Northwest Ordinance of 1787 considered the outstanding achievement of Congress under the Articles of Confederation?
I would say that this is because the Northwest Ordinance was very important in allowing the various states of the infant United States to continue as one nation. It also set the stage for further states to become parts of the Union. Both of these prevented things that could have been major problems for the young country.
Before the Ordinance, there were various competing claims by states on what was then called the northwest. If these claims had not been settled, the new states could have come into conflict with one another. This might have been fatal for the new country.
It is also important that a path to statehood was set up. This ensured that the areas of the northwest would not become colonies of the existing states. If there had not been a path to statehood, the territories of the northwest would have had grievances against the states -- they would have seen the states as oppressive and far-off rulers just the way the colonies saw Great Britain.
So the Northwest Ordinance did a lot to keep the young nation together and I think that is why it was the outstanding achievement of Congress before the Constitution.