In my mind, it is a toss up between Shikamaru and Naruto. Shikamaru is very smart, but he needs more techniques before he is a Kage level ninja. A lot more techniques. Yes, he took Hidan on one-on-one. But as Abdula said, Hidan only had one (very powerful) technique. I don't think Shikamaru would have had been successful fighting against Kakuzu, for example.
Naruto isn't as smart as Shikamaru, but he has a lot of powerful techniques, including the kage bunshin (which he uses very well), many variations on the rasengan, and call always call upon the Kyuubi in a pinch. Naruto has already been effective against kage level ninja -- Orochimaru, for example.
This is speculation, but I think Sasuke is ANBU. Assuming his is loyal to Konoha, he would be fine choice too.