What is the hardest artistic gymnastics move on the vault for a female?

This is theoretically infinite, but the most difficult vaults being performed today are the Yurchenko with 2.5 twist, or Amanar--round-off onto the springboard, back handspring onto the horse, 1.5 flip with 2.5 twists--and the Cheng, named after Cheng Fei--Yurchenko entry with half twist onto the horse, 1.5 flip with 1.5 twists (Rudi). Both vaults were performed by Cheng Fei and Hong Un-Jong at the Beijing Olympics, and others such as Shawn Johnson, Jiang Yuyuan, Jade Barbosa, Jordyn Wieber, Simona Amanar, and Monica Rosu have performed the Yurchenko 2.5. Though it is rumored that 2008 Olympic Vault Champion Hong Un-Jong of North Korea is capable of a Yurchenko triple twist, she has never performed it in international competition.