Kyoushoku Soukou Guyver (2005), Zetman Recommendations
Guyver was heavily influential for Zetman. Dark superhero series with similar premises and character dynamics; a fan of one is likely to enjoy the other.
The plots of the two revolve around powerful corporations responsible for the creation of monsters with the ability to disguise themselves as humans. Rivals of the leads in both are sons (adopted in Guyver) of the heads of the corporations and share hero/anti-hero relationships. And, of course, in both series full-body combat suits are used against the monsters.
Zetman would probably not exists if Guyver didn't. Both anime feature similar plot, gore, monsters disguised as humans and tragic events. Both heroes have similar powers which is somewhat biological and also like the monsters powers.
In both anime, the main protagonist can transform into a powerful humanoid being and must use their new superhuman abilities to project humanity from monsters that can morph into humans. Both anime also deals with a secret organization that wants to use their power for evil. Both are action-packed anime, filled with violence and gore. If you liked one, you'd like the other.
Monsters disguised as humans created by a secret organization and a a hero whit bio enhanced powers who fights whit them.
-Monsters disguised as humans
-Monsters are human made
-A suit which gives you powers
Both anime are series with the premise of monsters hiding inside human skin. Each consist of dark, grim stories about a protagonist (or in Zetman's case, protagonists) using superhuman abilities and technology to fight an evil organization of monsters. Each have major tragedy to the lives of all the main characters as the story unfolds, and the effects upon each character as it goes along. I personally enjoyed both series thoroughly and I think you will enjoy one if you like the other, although both Zetman and Guyver are small snippets of their respective stories, and left me wanting a bit more.