I'd place my bets on the ninja from Nabari no Ou, the likes of which Naruto stand no chance against, or any other ninja for that matter.
A powerful ninja, I think the second most powerful in the series, Yoite, can kill a person by shooting his energy into their body; since there are numerous ways to enter the body (mouth, nose, eyes, ears, groin, buttocks ect) this can be done almost instantly which gives off the impression that Yoite can kill by merely thinking it.
Next we have the hero, Miharu himself. He has a unique innate skill that he alone has from birth called 森羅万象, Shinrabanshō. The Shinrabanshō gives the user, Miharu in this case, the knowledge of the entire world, including jutsu. Now his personality is one that he rather would stay at home than to fight, so he wouldn't be going to any other world. If any other anime manga series were to venture into the world of Nabari no Ou, Miharu would instantly know their abilities.