Hehe well the show doesnt have to be about Naruto beeing the strongest, but about his goals, and who he is. Tho Naruto will probably be one of the most powerful things that exists in their world eventually. But if you look at the list to choose from, I'd say what i've said before it's impossible to say. Cause we dont know what any of them can do for real.

If it is meant to be up until now I think that Hokage (3rd) or maby Orochimaru are the strongest so far. And this Itachi guy we heavent seen alot more than his face, but they make him sound pretty badass.

1. Hokage.. I think they refer to him as one of the strongest if not the strongest Hokage. Tho he is getting pretty damn old he can probably pull of some crazyass shit if he has to.

2. Orochimaru.. one of the three, he is evil and snakey he uses strange jutsus (like the look in to my eye and see death kinda jutsu) when he fights and Hokage isnt worried about him for nothing I guess.

3. Itachi.. well dont know jack about him but he whiped the Uchiha clan out so that has to count for something

And then Naruto cause he gets a little better all the time and he has his Kyuubi.

And last Gaara if he is or can do what I think and hope he can do. Because possesions are alot different than a sealed thing. And I doubt Kazekage choose a lame spirit to make the greatest shinobi.. And that it all got out of hand has probably made Gaara even more dangerous than first intended.