It doesn't have to be the most powerful, it can be the most useful. A powerful technique can classify as useful. Anyone tell me if I am missing anything.
Chibaku Tensei for me, I think it's pretty Op. Just creating a massive moon. Nagato said he could even make it larger! And that was after he wasted a considerable amount of Chakra on the Shinra Tensei to destroy Konoha! That, and it literally was the technique to make the moon. THE MOON!
Creation of All Things, you can create whatever you want/need to win any battle ( which makes you as strong as you need to be, which also deals with the useful part ). It's pretty much the most versatile and effective technique in the Narutoverse.
I'll go with edo tensei, revive all the 4th hokages + edo madara, edo nagato, edo konan, edo J-MAN, edo haku, edo akatsuki's members, come at me bro! :O
Plot no jutsu has claimed so many victims. So many lives. So many OP individuals had to lose their lives simply to drive the plot further and for completely stupid reasons.
Deidara, Kakuzu, Kisame, Konan... Almost every akatsuki member. We will remember you.