1) Establish Justice... without justice, there would be no common ground for the others to sit upon. The justice is for the rules to abide by when setting up the others.
2) Insure Domestic Tranquility... along with justice, this would also fall into the court system and set grounds for the others to be held on common ground.
3) Provide For The Common Defense... this one is important, yes, but on the other hand, it has no common ground with the others to be established.
4) Promote The General Welfare... Along with the courts, this one also has to deal with the law and the wellness of the people, but then again, no common ground.
5) Secure The Blessings Of Liberty... For this one to really be set in motion, a court has to be set up.
Out of all of these, I would personally go with Establish Justice since it has the most common ground and is independent, also is the base of all the others.
Hope this helped!