According to Wikipedia if you count only cases in which only one dissent is filed and not joined by any other justice, even in part, then for the 21st century the answer is Justice Stevens. If you take the years from the 2000 sessions and exclude last years then he had 20 sole dissensions. Next is Thomas with 12, Scalia with 7, Ginsburg with 6, Breyer with 5, Souter with 3, Sotomayor with 2, Alito with 2, Kennedy with 2 and Rehnquist with 1.
That’s not really the same thing (unless you mean nobody else joined in the dissents.)