I'd suggest One Piece by Eiichiro Oda. It's rated 13+, but there's really nothing bad in it beyond minor swearing and a fair bit of violence. It's about a boy named Luffy who wants to be the king of pirates. Yes, this manga does promote piracy, but the main characters are "good" pirates who wont stand for injustice and are constantly whooping "bad" pirates' butts. The only other controversial bits would be that Luffy ate the cursed Gum-Gum fruit, often called the fruit of the devil which gives him supernatural powers but disables him from ever being able to swim.
Most of the violence takes the form of Luffy's comical punching/kicking/stretching moves, and don't inflict any gory damage aside from the occasional bloody nose or mouth. Zoro (another protagonist) fights with three swords, but nothing close to explicit. In fact, most of the fight scenes are rather cartoony and comical.
As I said before, the swearing is very minor. An occasional hell or d***, but that's it. The morals and comicalness of this manga vastly override any "flaws", I think. It's one of my very favorites!