No one is a match for the raw power of the Kyuubi, no one.
The 4th didnt actually fight the Kyuubi he just sealed it

no matter how good Itachi is with one stomp of the Kyuubi in his actual form Itachis dead, have to remember that the Kyuubi isnt a nin or anything, hes a monster and hes the size of a stadium.
if Naruto ever gets 100% of the Kyuubi power theres no one that'll stop him or even come close