Itachi.

Itachi mastered the Sharingan, meaning three comma and complete usage of its abilities, at the age of eight. He was Chuunin the following year, thus at nine. And, ANBU Captain at thirteen. He's up there. I don't see them handing over such a prominent position to just any pre-pubescent shinobi. Itachi was greatly skilled, and heralded, as well as feared, by many. Rightfully so.

Akatsuki have been running around doing their thing in acquiring power and setting things up for their master plan. In all that time, you seriously doubt Itachi's arsenal of techniques and skills doesn't compare to Orochimaru's? I'm sure Itachi's seen just about every jutsu from when he was still a child. He got bored with copying, replicating and improving the same skills, thus wanted more. Enter Mangekyou.

But, even before Mangekyou, Itachi was the ANBU Captain. You don't have to be skilled or smart for that position?

There have not been many Ninjutsu skills shown by Itachi. But, with the insanely swift speeds with which he executed hand seals, do you really doubt him? Kakashi barely caught them. I do not think Kurenai did catch them.

You also mention Orochimaru's got years on Itachi. That isn't necessarily a good thing. He's out of his prime. Still a force to be reckoned with. However, I'm sure that's wavered some since his younger years.

In addition, I think Itachi's familiar with Orochimaru's technique. Why? Well, how do you know something is hot? You touch it, and that unpleasant burn is something you remember. How would Orochimaru know he could not defeat him? He's most likely attempted to do so before. Unable to best the premiere Uchiha, Orochimaru decided to go for the untapped one, Sasuke.