Most Overrated USSC Justice of the 20th Century,
not including any Justice who has served in the 21st century. My tentative vote goes to Louis Brandeis, who also wrote the most overrated dissent of the 20th century, his ridiculous pro-monopoly opinion in New State Ice v. Liebmann. But I'm open to suggestions, below.
UPDATE: For a partial explanation of my nomination of Brandeis, see the Comments.