Surely, Alluka is a beast when comparing Nen abilities, but such an ability is too op for me so I won't include it in my tops. My favourite Nen ability is Feitan's Pain Packer. We've only seen one form of it (Rising Sun) which left me speechless. The very concept of empowering one's ability the more damage he has been dealt is genius. I am so eager to see what other forms Pain Packer can achieve if Feitan gets severely or even fatally injured. Don't think there could be anything stronger than that, putting Alluka's ability aside.