Definitely Nocturne's. When your jungling or in lane and you see the enemy Nocturne ult, you keep wondering who he saw and who he going to get. You wonder if he sees you via ward or if hes trying to gank your team. Either way, you dont wanna move until its over, then you try to help the poor souls getting ganked. Even worse is when the enemy team has a TF and a Nocturne, then, when they decide to ult, you know you are screwed.
That, or Darius'. Cuz you know, that even if you are winning against a lvl 6+ Darius, you are going to lose.