By looking at the series, only ones that can really be considered strongest are the ones that are truly holding to their convictions. Very few fall into that, is why they have Naruto sticking to his through the entire series to show him gaining strength through the ordeals he goes through. But on all the new Akatsuki stuff and the 4th being so powerful. Well if you really looked at it, chances are Akatsuki's leader is the old leader of the village of the sand who disappeared. The 4th is actually Naruto's father and is why he sacrificed his own son rather than a random member of the village's. And so forth, so really, its more a matter of the current strength to the strength they gain as time goes on. The ones that have the demons sealed within them are by far the strongest if they can learn to use their powers. As their are a total of 9 of them. The strongest will be presented most likely as one of them. Though as Akatsuki takes them from the people that have them within. Their already impressive strength becomes even stronger. Though the leader being a previous Kage is most likely to be the first to take their powers into himself and become even more powerful as to one day have to be a huge battle for Naruto to become more power than. Unless the series is to end, in which case they'd most likely make Naruto have a family and become Hokage making him the strongest and end it there. So to conclude whats now become a rant, its Basically series revolves around someone else always being stronger and over coming them, in the end though, the strongest always has to be the main character. So Naruto.