In my opinion, I would have to say the Shadow Possession Jutsu. Now, you probably wondering why I wouldn't say one of the hokage's ability's. I figure that out of all of them, this ability has more uses. Not only can you use it against the enemy, but you can use it on a team member who may be unable to move and there fore move them with you Jutsu in order to save their life. Also, with well thought out planning this ability could be used to it's fullest. However in the end I believe that none of the Jutsu are at it's greatest without a good combination of others as well. If you have a good team, and a broad amount of Jutsu used by each team member, you have the upper hand against any single Jutsu alone.