Toaru Kagaku no Accelerator

Both shows have an overpowered male lead and a childish looking female lead. Both shows are part of the supernatural and mystery genres. 

Both are similar in the sense that they're both spin-offs about the most interesting character from their own respective series but they were somehow put into really uninteresting stories that leave much to be desired.  

This recommendation is one-sided since "Accelerator" is part of a massive franchise (which means you need to watch the series before it to understand this one) unless you have already seen/know the "Toaru" franchise. In simple terms, if you enjoyed the premise in accelerator wherein a powerful being is babysitting a literal child, then you might want to check "SomaliMori." Apart from that, I think the two are similar (aside from the premise) in retrospect of their character dynamics while being different in everything else (especially in the concepts used). With that said, I find "Accelerator" being fairly similar to "Golem" in terms of their attributes, while "Somali" having the capacity to bring the same vibes as "Last Order" as they essentially have the same personality. read more 

In both series, the protagonist is extremely powerful who is nearly invincible and have crazy personality. Both series also have gore and zombies as well. 

Zack and Accelerator are both psycho, powerful and have the same voice actor. Both anime are dark and gory. 

My favorite part of the Toaru and Isekai Maou series! Accelerator the OP crazy has his main series where he's the main! Accelerator and Diablo both show off their power if you like badass and impress to keep their maniacal/demon lord image in front of cute girls, I'm referring to ESTHER IS HOT LUMACHINA IS HOT !!!!!!! 

The story is focusing on "Levi" humanity's strongest and "Accelerator" Academy City's strongest.