As of my knowledge cutoff in June 2023, the following are five of the most famous manga series worldwide and the reasons for their popularity:

1. "One Piece" by Eiichiro Oda: "One Piece" is a long-running adventure manga that has captured the hearts of millions of readers around the world. It follows the journey of Monkey D. Luffy and his pirate crew as they search for the ultimate treasure, the One Piece. The series is known for its compelling storytelling, vibrant characters, epic battles, and intricate world-building. Its themes of friendship, dreams, and the pursuit of freedom have resonated with readers of all ages.

2. "Dragon Ball" by Akira Toriyama: "Dragon Ball" is a classic manga that has had a significant impact on the genre and popular culture. It revolves around the story of Son Goku, a Saiyan warrior with superhuman abilities, as he trains and fights against various powerful adversaries. The series is renowned for its intense action, memorable characters, and its ability to blend martial arts, science fiction, and comedy. It has spawned numerous anime adaptations, films, and merchandise, solidifying its global recognition.

3. "Naruto" by Masashi Kishimoto: "Naruto" tells the tale of Naruto Uzumaki, a young ninja with a dream to become the strongest ninja in his village. The series explores themes of friendship, hard work, and overcoming obstacles. It captivated audiences with its engaging characters, intricate plotlines, and intense ninja battles. "Naruto" has a dedicated fanbase and has garnered widespread acclaim for its storytelling, character development, and emotional depth.

4. "Attack on Titan" by Hajime Isayama: "Attack on Titan" is a dark fantasy manga that takes place in a world where humanity is on the brink of extinction due to the existence of massive humanoid creatures known as Titans. The story follows Eren Yeager and his friends as they join the fight against the Titans and uncover the mysteries surrounding their existence. The series gained immense popularity for its gripping plot twists, complex characters, and thought-provoking exploration of themes such as morality, survival, and the nature of power.

5. "Death Note" by Tsugumi Ohba and Takeshi Obata: "Death Note" is a psychological thriller manga that revolves around Light Yagami, a high school student who discovers a supernatural notebook that grants him the power to kill anyone whose name he writes in it. The series delves into themes of justice, morality, and the nature of power as Light uses the Death Note to carry out his vision of creating a perfect world. "Death Note" became famous for its intricate cat-and-mouse mind games between Light and the enigmatic detective L, as well as its exploration of ethical dilemmas.

Please note that manga popularity can vary over time, and there are many other exceptional manga series that could be considered among the top in the world.