That Time I Got Reincarnated As A Slime vs Overlord

Connection:

Both are normal people who got Isekai-d into another world and turned into another species

Both are are very overpowered

Both also have harems

Both gained the titles of demon lords

Both ran a country comprised entirely of monsters

Both also killed humans in the thousands though their reasons are different (Rimuru did it for revenge and Ainz did it to rule YGGDRASIL)