Books: Ranma 1/2
The very first manga I ever purchased was "Ranma 1/2" by Rumiko Takahashi. It's about a guy who turns into a girl when he's splashed with cold water, his tomboy fiance Akane, and all their crazy friends/enemies. "Ranma 1/2" is the sort of series that gives reviewers fits - at times it's about as slapstick and scattershot as they come, but every so often a Serious Moment intrudes on the action and Ranma and Akane seemingly get closer to declaring their love for each other...seemingly.
The art is often a bit repetitive, as are the plotlines, but the characters and their interactions are broad enough to sustain the silly premises thrust upon them. It's almost as if Ranma and Co. are so caricatured that they become amenable to almost any emotion and situation - kinda like real human beings. Kinda. In the end, the series does have lots of genuinely funny gags, and the martial arts action is rarely unimaginative, so it really appeals to everything a thirteen year-old boy could ask for.
If there's one thing Ms. Takahashi is terrible at, it's ending a series promptly. Ranma has literally dozens of 200-page volumes, and Inu-Yasha, Takahashi's current project, is going past 50 volumes. Whether it's milking a cash cow, giving fans more of what they want, or just simply exploring what Takahashi wants to explore, when you start reading one of these series, get ready for the long haul.