For me, 'Parasyte' comes to mind. It has similar elements of mystery and a darker tone like Drug Candy. Also, 'Death Note' could be considered somewhat similar with its complex characters and intense plot.
You might like 'Orange' as it also deals with complex emotions and relationships like 'Drug Candy' does. It has a similar style of exploring the inner lives of the characters. Another one could be 'Koe no Katachi' which shares the theme of personal growth and understanding.
You might like 'Soul Cartel'. It shares some elements of mystery and drama like Drug Candy. The character development and plot twists can give you a similar engaging experience.
The similarities could lie in the way the stories are structured. They might have a build-up of tension and resolution, or use similar plot devices to keep readers engaged. Also, the level of detail in the backgrounds and the way the scenes are composed could be comparable.
Some mangas similar to Punpun could be 'Goodnight Punpun' and 'Solanin'. They share similar themes of coming-of-age and emotional depth.
For manga similar to Nisekoi, you might consider 'Ore Monogatari!!' and 'We Never Learn'. They feature cute and charming characters and deal with various aspects of love and relationships in a light and enjoyable way.
One manga that could be similar is 'Shinigami no Ballad' as it has similar themes of strategy and mystery.
Well, some manga similar to Dandadan could be Jujutsu Kaisen and Chainsaw Man. They have similar elements of supernatural powers and action.
For manga similar to Gokusen, you might consider 'Sakamoto desu ga?'. It has a similar comedic tone and features characters with unique personalities in a school setting.
Well, 'Solo Leveling' has some similarities in terms of action and fantasy elements. It also features a protagonist's growth and challenges.
I think mangas that lack creativity, have repetitive storylines, or fail to engage the readers' emotions could be seen as similar to trash. It really depends on individual preferences and expectations though.