I think Itachi is an all time great in Naruto, hes def in the top 10. Itachi is a character that we will never see his true potential. Every time we see him in action he always has new abilities shown for that situation or some unknown knowledge on the situation. I think Kishi did this on purpose so he can still have what if "hype". Very similar to Minato. If you think about it, Kishi needed Itachi to die. But he wouldn't let Itachl be defeated in an all out 1v1. So he made Itachi sick so that he can die by his own hand basically. From what we have seen and his abilities on paper, he is def top 10 in Naruto.