Itachi is stronger than Orochimaru, where Orochimaru's strengths lie. That's probably in all forms of taijutsu, ninjutsu and genjutsu. So, as far as being a skilled ninja, and fighter, Itachi is above.

But, Orochimaru might have more insight, and judgement. The man is a lot older, after all.

Somebody add the "Who's stronger than who?" topic to Kakashi's repeated questions sticky.