User blog comment:Prince of Erebor/Weakest Nen ability/@comment-14902757-20151018003714

Quite a tough one coz like what others have already suggested, any nen ability can be considered weak when the conditions are unsuitable for its use but could be an ace in someone's pocket if it meets all the right conditions.

For me, Leech's ability is the weakest. When it's used in a battle, the condition for using his ability can only be met by first inflicting a wound on the enemy and he has to directly plant the leeches while the enemy lays still. That or maybe it's really a torture ability and not a fighting ability.