It's not possible to categorically identify the greatest. However, the following Japanese superheros have had a huge cultural impact.
1. Ultraman (ウルトラマン)Ultraman isn't a single superhero: there are hundreds of Ultraman collectively known as Ultra Crusaders. They are super-humanoid solar powered giants from space who defend earth from giant monsters and aliens.
Ultra Crusaders have numerous superpowers. Their one weakness: they can only stay on earth for 3 to 5 minutes before their power runs out. They must regularly return to space to recharge their batteries.
There have been over 20 Ultraman TV shows (series) and over 20 Ultraman movies since the mid 1960s.
2. Kamen RidersKamen Rider is a long running series of manga, television programs and movies. Like Ultraman, Kamen Rider isn't a single superhero. Instead, there are over 100 Kamen Riders. All are grasshopper-like superhumans who ride motorcycles.
3. Super Sentai (Power Rangers)Super Sentai is another superhero series with dozens of TV series, movies and characters. They are better known in the English speaking world as Power Rangers.
4. Sailor MoonSailor Moon are a team of teenage girls who can transform into heroines named for the moon and planets.
5. Metal Heroes (Space Sheriffs)Metal Heroes is a superhero series featuring a variety of androids, cyborgs and humans who wear metallic suits.
6. Astro Boy (鉄腕アトム)The first Astro Boy manga hit the shelves in 1952.
Astro Boy takes place in a future world in which robots and humans live side by side. Astro Boy is an advanced robot with the ability to express human emotions. He defends humans from robots gone berserk, human-hating humans and alien invaders.
7. Science Ninja Team GatchamanFive young superhero ninja who protect the World's natural resources from giant monster robots.
8. Mazinger ZMazinger Z is giant super robot constructed of a metal called Alloy Z. This super metal is made of a new element (Japanium) found only deep inside Mount Fuji.
9. Pegas (Tekkaman)Pegas is a Tekkaman — a super strong human with a suit of indestructible armor and a variety of super weapons and tools.
10. Honey KisaragiA 16-year-old girl who discovers she is a super android.
11. InazumanA mutant in armor who can absorb the kinetic energy of any attack — it charges his biomechanical batteries.
12. Japanese SpidermanIn 1978, Spiderman was licensed by a Japanese TV production company. A total of 41 episodes were produced. As you might imagine — it was very different from the American version.
Japanese Spiderman has a giant robot called Leopardon he controls.
13. Gekko Kamen (Moonlight Mask)Japan's top superhero of the 1950s. Gekko Kamen had a secret identity (it's never revealed who he is). The actor in the Moonlight Mask TV series and movies is only credited with a "?".
The Moonlight Mask wears a turban over his face. He's armed with a ninja star (shuriken), whip, two six-shooters and two boomerangs. He rides a motorcycle.
14. Ogon Bat (Golden Bat)Ogon Bat was first published as a Japanese pulp fiction way back in 1930. He's the first Japanese Superhero.
Ogon Bat is a protector sent forward in time from Atlantis. He has a staff that can cause earthquakes or shoot energy bolts.
15. Cyborg 009A team of superheroes led by Joe Shimamura — a delinquent half-Japanese teenager. Joe is a cyborg who can move so fast that everyone else appears to be perfectly still.
16. Tetsuo Shima (Akira)A human with immense psychic powers. He has mental problems that get worse the more he uses his abilities.
17. MegalomanA guy who studies martial arts and is secretly from another planet giving him superhuman strength.
18. Super Robot Red Baron (スーパーロボット レッドバロン)An evil organization steals all the World's super robots at one time. Only one super robot escapes: Super Robot Red Baron.
19. MirrormanA photojournalist named Kyotaro Kagami discovers a secret from his past: his father is an alien superhero around another dimension called Mirrorman. His mother was a normal human. Therefore, Kyotaro Kagami has only half of Mirrorman's superpowers. He must use them to defend earth from alien invaders.
20. KikaiderA Japanese superhero from the early 1970s who was (strangely) more popular in Hawaii than Japan. Kikaider is a humanoid robot with a conscience circuit.