My instinctive reaction was that Lanfear should be the worst one on the list, though good arguments are here for why Semirhage and Mesaana are worse. Lanfear was very self serving, to my mind did little for the 'cause', and even for herself just seemed to have one spectacular fail after another. She made for good reading, and did add good suspense at times, but the threats were never really realised.