Itachi is insalenly powerful as we have seen him fight against Sasuke and he was very ill and Madara said he still cud of killed Sasuke. Itachi is also very talented in many ways he is also very intelligent, but Sasuke himself is a powerhouse now and has EMS! But we are seeing even more powers from Itachi currently. Who wud win if Itachi was healthy and both were fighting each other to kill!