DragonBall

This is not a new manga and dragonball is often considered as outdated but it is one the best mangas for many reasons: it's an unpretentious story and as the comics, mangas are mean to be simple and powerful; dragonball creates a complete and robust universe and finally the story and the characters always move forward and progress, there is no wasted time or stagnation in Dragonball. You can like it or not, DragonBall is still the first BIG manga, long before Naruto and the others, and it was the first manga i read, the one that made me love manga!
The story: a young boy living alone meets a girl looking for some strange balls, the dragonballs. He decides to help her in her quest and turns out to be a very strong fighter in martial arts. During the journey they make a lot of friends and together they happen to save the world several times.

edit: A DragonBall movie is going to be released early 2009, you can find an extract there.