I had heard that the top special rune drop for the Countess is Ist, but her "normal monster" drop can be up to Vex. . .
In any event, I just killed the Countess and had her drop a Jah rune. This sort of surprised me, since I thought this was impossible. . . which leads me to the question -- what are the top runes the Countess can drop and the relative chances of each? Or maybe a nearby minion dropped the Jah, but I would have presumed them to be a lower level than the Countess herself. . .