I give Naruto a 10 because it has stayed really, really good for a very long time now. How many other mangas are still entertaining you after 400 episodes? (Not Bleach, that's for sure.)
Lots of new mangas start out strong. You have fun exploring the new world the artist created; you get tantalizing hints about deeper conspiracies and hidden pasts of the different characters; maybe an unexpected plot twist throws you for a loop. In the end, though, I usually wind up disappointed. Once the first set of ideas have been used up, all of the sudden I'm being introduced to a whole new set of generic villains, being asked to care about the backstories of a batch of replacement sidekicks, or watching the same old conflict from volume one drag on and on because the writer doesn't know what to do once it's resolved.
With Naruto, it never feels like Kishimoto is dragging things out just to collect another paycheck. He's never out of ideas. New characters always feel like they have roots in the world of the story and believable connections to what came before. Naruto develops, but he stays the same person at heart. Because I have confidence in the storyteller's talent, I can suspend my disbelief and really get into the story.
(I also think that Kishimoto would kill off any and all of his favorite characters if that's what the story demands. Compare that to Bleach, where the good guys never die and always get healed completely. When the characters are supposed to be in "danger", how am I supposed to care? I know they'll be fine.)
Yeah, you can argue that other manga have more "depth," but great storytelling is a talent that's handed out to very few people in a generation. Today, we read Dumas and Dickens: the 19th century's great storytellers, not its great philosopher-authors. In another hundred years, people will probably be reading Steven King and J.K. Rowling, not David Foster Wallace.
Naruto is great storytelling. That's a good thing!