S+ (Strongest in each class)
* Yami Yami no Mi
* Gura Gura no Mi
S (Admiral Class)
* Azure Dragon
* Magu Magu no Mi
* Goro Goro no Mi
* Hie Hie no Mi
* Pika Pika no Mi
* Gravity Fruit
A (Top Commander class fruits)
* Mera Mera no Mi
* Ito Ito no Mi
* Phoenix
There are so many Devil Fruits, but I'm not sure what the other tiers below them would be.