The way Zoro emphasized it, by saying "it was one of my strongest moves", I doubt it isn't even top 3. It is a finisher move based on high lethality. Using it to one shot Daz through his super hard iron defense, using it to behead the Dragon, using it against Kuma, using it against Apoo as he had enough and got serious.
Shishi Sonson is definitely top 3.