Itachi definitely.
-Full use of both Sharingan and Mangekyou Sharingan (Unlike Kakashi, who isn't a full Uchiha, can't use his eyes to their full potential)
-Genjutsu Prodigy (Could cast Genjutsu by merely pointing, and could turn a Genjutsu back to the user with ease. Could understand the concept of Izanagi and could use Izanami
-Battle prowess (When he and Kisame came looking for Naruto after the Chuunin Exams they defeated Kakashi, Kurenai & Asuma, three of Konoha's top jounin and Kakashi admitted Itachi wasn't even using half his power)
-Mastered Mangekyou Sharingan despite side effects.
-Managed to fight Sasuke for a good while even though his sight had deteriorated a lot
While Kakashi is a powerful character having graduated from Academy at 5 and become Chuunin at 6, once he got his Sharingan from Obito he became to reliant on it stunting his growth and preventing him from making the leap from A-rank to S-rank status. That's why I believe Itachi surpassed him a while back.