Yoru is the strongest blade because it is the only one out of the 12 to turn black. I think any of the other 12, if they turned black, would be the strongest blade. Maybe it's as simple as that.
Kozaburo, the legendary blacksmith, mentioned to Zoro that Enma could transform into a Black Blade over time. If Enma were to become a Black Blade, it would be classified as one of the Supreme Grade Swords, and the total number of Supreme Grade swords would increase from 12 to 13. The question is that among the top 12 swords, there are weapons that are not black, so by turning black, their classification would increase, potentially surpassing even Yoru.