Hard to tell. Sydney is the chosen one and she's hell of a spy but though we didn't have the chance to see Jack on action as much as Syd, he frightens his opponents just by a look of his. He and Irina have the auras of a perfect spy but Sydney grew up to be like them, she was more like her parents in season 5.
Who would win in a fight against each other, Nadia or Vaughn?