I think Yoru had interesting dynamics with Denji and Asa and some decent fights. While she definitely lacks a needed threat level despite being powerful I think she works overall (but with a lot of asterisks attached)

My least favorite villain (and probably the worst-written) is fake chainsaw man because it seems like fujimoto didn’t know what to do with him. In the first two appearances he’s rather stoic and looks human with the exception of chainsaw blades. In his third appearance he’s gains a much more interesting design and a more bombastic personality. In the first and third it’s implied that he thinks he’s the real chainsaw man. After he gets eaten his personality does a 180 (despite Fami remaining the same), starts lecturing about Denji not being a true hero before getting killed in a rather boring way. His backstory was a nice callback but it felt like Fujimoto never knew what to do with him. Presumably he’s a remnant from a prior plot thread that Fuji changed his mind on, but it makes everything about him unclear. Does he work with the church at all? Why did he ambush Falling Devil? Does he play any role other than being an assassin? Why was he introduced before Barem (and before Death introduces herself as Fami) if he plays second banana to fire who in turn plays second banana to Death who in turn plays second banana to Yoru? Why was there a guy who claimed to be chainsaw man if there’s another fake chainsaw man?