EL-wise, a single 15th lvl character is supposed to be as tough as a party of four 11th lvl ones, so I'd say an 11th lvl module will do.
That being said, let me just add that on a solo adventure, levels don't matter that much. There will be a lot of things that the 11th lvl party can breeze through that the 15th lvl guy cannot, simply because he individually can't bring as varied a set of abilities to the table as the group. Also, a single PC, however high-level, has the huge Achilles heal that there is no backup. One failed save at the wrong time and he's dead, since he has nobody to keep the enemies off him. If he's bringing no NPCs or henchmen with him, he'd better play very smart and cautious.