Please note that part of this is influenced by my tastes in manga. If you don't see a great author I merely missed him.
Osamu Tezuka (1928 1989)
Also known as the “God of Manga”, Tezuka was an eclectic manga author and anime director. His works range from Kimba the White Lion (the story of a young lion that is influenced by human culture), to the Shojo manga The Sapphire Princess (a series that follows a girl ruling a kingdom disguised as a man, and her children. It's also one of the first manga aimed to Young girls) to Black Jack (that shows the story of an unlicensed doctor that can cure, is rumored, any disease) to Budda (a story of the life of Siddharta Gauthama) to Story of Three Adolph (a story following the life of a jew, a German and the dictator, considered his masterpiece).
Go Nagai (1945 - alive)
Go Nagai is one of the most prolific authors in manga history. His works introduced the Mecha (see his Mazinga series) and erotic (with his work Harenchi Gakuen) theme in the manga world. He also wrote two manga directly influenced from Dante Alighieri’s Divine Comedy, Mao Dante and his masterpiece Devilman, and a manga version of the Comedy. And this is a fragment of his enormous production.
Eiichiro Oda (1975 - alive)
Eiichiro Oda is on the opposite side of the spectrum: his only work, One Piece, has however already influenced a whole new generation of mangaka, and after twenty years is growing strong.
Akira Toriyama (1955 - alive)
The creator of Dragon Ball, Akira Toriyama influenced the shounen world for an Entire generation. His other works include Dr. Slump, a fine example of gag manga, and many other shorter works.
Rumiko Takahashi (1957 - alive)
The “Queen of manga”, Rumiko Takahashi wrote some of the most famous romantic manga, but at the same time managed to d'età che herself from the canon of Shojo and appeal to both male and female readers. Her first work Lamù is a great opera, and so are Ranma and Inuyasha.
Takehiko Inoue (1967)
Takehiko Inoue is one of the most influential spokon (sport manga) authors. His manga Slam Dunk, Buzzer Beats and Real (the last one about the hard theme of wheelchair basket) brought a new generation in this sport. And he is also the author of Vagabond, a manga about the life of the greatest ronin samurai of all times, Musashi Miyamoto).
Toru Fujisawa (1967 - alive)
School life manga are a great deal in Japanese comics, and the greatest of them all is Great Teacher Onizuka, Fujisawa masterpiece. The protagonist is however original of his first manga Shonan Junai Gumi, where we read about the infamous runt Onizuka, that will later become the before mentioned Great Teacher. More and more spinoff are created by Fujisawa, always great.
Hiromu Arakawa (1973 - alive)
The creator of Fullmetal Alchemist, Hiromu Arakawa work are a great example of storytelling and world building. She also wrote the manga Silver Spoon, a story about the daily life of an agrarian School student, and is the drawer of The Legend of Arslan manga.
Kentaro Miura (1966 - alive)
Mainly known for his seinen manga Berserk, he wrote many more manga with his carachteristic gore and great story telling, for example Gigantomachia.
Yoshihiro Togashi (19 - alive)
Famous at first for his manga Yuyu Akusho, Togashi gained fame for his manga HunterxHunter . However, his shorter works also deserve merit.
Honorable mentions
Kouta Hirano (Hellsing and Drifters);
Nisioisin (Medaka Box);
Kohei Horikoshi (Oumagadoki Zoo and My Hero Academia);