If you liked
Riki-Oh
|
...then you might like
Black Angels
|
Heavy religious themes, questionable and weird 2nd half a main character that goes through a great deal of loss and self discovery, weird fighting styles and bizarre villains in a dystopic setting that and many other things make Black Angels similar to Riki-Oh, as a matter of fact, the artist for Riki-Oh was an assistant to Shinji Harimatsu and worked on Black Angels.
If you liked
Sakigake!! Otokojuku
|
...then you might like
Osu!! Karate-bu
|
Both series are about manly delinquents using bizarre martial arts to fight their opponents, both series also start as gag manga but slowly shift their focus into action, following the trend of popular battle shonen at the time
If you liked
Hokuto no Ken
|
...then you might like
Koukou Tekkenden Tough
|
Both manga have a main character that is the heir to a brutal martial art, and have to honor their friends in battle and fight several other martial artists.