Lawyers who practice in federal court will likely remember Justice Souter best/worst for writing the Court's opinion in Bell Atlantic Co. v. Twombly, 550 U.S. 544 (2007). Twombly held that a complaint can't survive a motion to dismiss unless it states a "plausible" claim. It also retired the Court's old shorthand for judging a complaint — the "no set of facts" gloss from Conley v. Gibson, 355 U.S. 41 (1957). Courts cite Twombly more often than any other case.
The pro-defense Twombly outcome contrasts with Justice Souter's generally moderate-to-liberal record on the Court.