Batman v. Commissioner, 189 F.2d 107 (5th Cir.), cert. denied, 342 U.S. 877 (1951).
United States ex rel. Mayo v. Satan and His Staff (W.D. Pa. 1971).
A reader just sent me the excellent "Batman v. Commissioner," of which I wasn't aware until now. (Yes, technically it should be Batman v. Commissioner of Internal Revenue, not Commissioner Gordon, but I'm going to let that slide because it's making me laugh as is.) Turns out that another case by that name was decided in Iowa in November 2009, but I guess in situations like this I will cite only the earliest of the cases. (It is important that we have rules to govern these kinds of things.) Please feel free to send me your submissions as well.
Of course, Mayo v. Satan is already in the Case Law Hall of Fame, but it probably belongs on this list, too, which is for cases that have comical names whether or not the substance of the opinion itself is amusing. (Again, rules.) A case against Satan is not unlikely to end up on both lists, but it depends.