Seiken Gakuin no Makentsukai

S: Both MC transport to another world Both MC have overpowered skills Both MC have female group (harem) Both MC are cold/hide their emotion D: 100-man is gaming world vs Seiken is fantasy world 100-man is modern day MC vs Seiken is fantasy day MC 

S: Both MC are reincarnated as kids Both series take place in fantasy worlds Both MC have overpowered skills Both MC try to hide their skills Both MC have female group (harem) D: Seirei MC was human vs Seiken MC was monster Seirei MC is from modern day vs Seiken MC is from fantasy world 

S: Both MC are reincarnated as children Both MC hide their powers Both MCs have overpowered skills Both MC have female groups (harem) Both series are set in fantasy world D: Sekai is modern isekai v seiken is fantasy isekai 

S: Both MC reincarnate as kids Both series take place in fantasy world Both MC have female group (harem) Both MC hide their overpowered skills D: Hachi reincarnates from human vs Seiken reincarnates as monster Hachi MC is from modern day vs Seiken MC is from alternate fantasy world 

S: Both MC transport into fantasy worlds Both MC have overpowered skills Both MC have female group (harem) D: Seiken MC reincarnates vs Smartphone MC isekai Seiken MC was monster vs Smartphone MC was/still is human 

Similar plot, reincarnated OP MC meets harem.  

S: Both MC are reincarnated into fantasy world Both MC have overpowered skills Both MC try to hide their abilities Both series have female group (harem) D: Overlord MC reincarnates as monster vs Seiken MC reincarnates as human 

S: Both MC are reincarnated as children Both MC have overpowered skills Both series are set in fantasy world Both series have female group (harem) D: Knights has mecha vs Seiken has advanced tech Knights is modern isekai vs Seiken is fantasy isekai  

main protagonists are demon lord/death god who got killed by humans in their era and reincarnated into the body of a young boy in the future and learn to adapt and relive their life