Itachi... As much as I hate Sasuke's PERSONALITY, there is no denying he has become VERY strong and, actually, always has been. However, Itachi let Sasuke win and was sick, and Sasuke barely won, so if Itachi went all out from the start, Sasuke would have easily lost.
I hate to say it as itachi is my favourite character, but i think susuke is the stronger siblling now. sasuke has a weapon in the eternal mangekyou sharingan that itachi never had. sasuke has never lost a fight either. it says a lot when you manage to always pull through, no matter what the circumstances
its also important to note that kishi has set sasuke up to be the equal in power to naruto, so you have to ask yourself who is more powerful out of naruto and itachi, as we havent seen sasuke fight in ages