I dunno, Itachi is good, but KB managed to pretty much kill Sasuke like 3 times in a fight, and he even broke out of Tsukiyomi, even if it was weak, so Tsukiyomi can't be an instakill. Sword of Tesuka or whatever it's called is good, but Itachi used it on a slow ass 7 branch giant Orochimaru, I doubt he could hit KB just like that. Susanoo also takes a heavy toll on Itachi so unless he could end it quickly, I'd say Killer bee gets it. If Itachi was healthy I dunno, but MS is so damaging to the user, I think KB gets it, Sasuke used Amaterasu on him and he escaped, so Amaterasu isn't an instakill, so yeah, KB gets it