It's hard to pick one as the worst, but one case that does spring to mind is
Allgeyer v. Louisiana. This terrible decision (as a matter of constitutional interpretation) was the precursor to the
Lochner era. The absolutely awful opinion includes a circular argument at the crux of its decision.
As so construed, we think the statute is a violation of the Fourteenth Amendment of the federal Constitution in that it deprives the defendants of their liberty without due process of law. The statute which forbids such act does not become due process of law, because it is inconsistent with the provisions of the Constitution of the Union. The "liberty" mentioned in that amendment means not only the right of the citizen to be free from the mere physical restraint of his person, as by incarceration, but the term is deemed to embrace the right of the citizen to be free in the enjoyment of all his faculties, to be free to use them in all lawful ways, to live and work where he will, to earn his livelihood by any lawful calling, to pursue any livelihood or avocation, and for that purpose to enter into all contracts which may be proper, necessary, and essential to his carrying out to a successful conclusion the purposes above mentioned. ...
To deprive the citizen of such a right as herein described without due process of law is illegal. Such a statute as this in question is not due process of law, because it prohibits an act which under the federal Constitution the defendants had a right to perform.
The italicized part is the circular argument; what makes it circular is the fact that the due process clause was the only provision in the Constitution the Court invoked.
Another infuriating aspect of the Court's opinion is the huge amount of ambiguity behind how much legislative power the Court would allow state legislatures to exercise: "... we do not intend to hold that in no such case can the state exercise its police power. When and how far such power may be legitimately exercised with regard to these subjects must be left for determination to each case as it arises." So no state legislature can know whether the powers it tries to exercise might be, or might not be, acceptable to the Court.
