This topic again?

The answer is Itachi. The reason Itachi has retreated twice so far is because he's smart enough not to risk a fight with a very powerful opponent (or large number of opponents) when it isn't necessary. He doesn't need to fight Jaraiya, so he isn't going to if he can avoid it. Even if there's only a 10% chance that Jaraiya will win, that's still a risk. That intelligence itself is a powerful asset.

Also, think about it thematically. Sasuke is going to be one of the strongest characters in the series by the time it ends, probably second only to Naruto. They're the main characters in the series, with Sauske being Kishimoto's favorite. Given the genre, they are most likely going to surpass the Sannin. His last fight is going to be with Itachi (it's his character's main motivation; so it stands to reason that it will be resolved at the end of the seires). His fight with Itachi is going to have to be dramatic, so Itachi will have to be stronger than Sasuke, with Sasuke comming from behind to barely win. Therefore, Itachi is going to have to be stronger than the Sannin.