To determine this fairly, we'd have to rank each Slayer Magic on magic alone and not by the user who performs it, as it can vary tremendously.
Dragon Slayer
Standard Slayer Eating Ability
Dragon Force (Can be Forced by Willpower)
Can NOT Eat God Slayer Elemental Magic
God Slayer
Standard Slayer Eating Ability
Black Coloring added to Techniques
No known upgraded form
Devil Slayer
Standard Slayer Eating Ability
Arguably uses Curses as opposed to Eternano (Highly unlikely)
No known upgraded form
Dragon Slayer Fire Magic has been shown to burn through standard Ice Magic, but has no effect on Devil Slayer Ice Magic. The fact it offers no effect, not simply being weak to, no effect, implies Devil Slayer Magic is stronger than Dragon Slayer
Devil Slayer can also, implied at least, defeat Devils, better known as Demons, while Dragon Slayers don't seem to be able to unless entering Dragon Force.
Dragon Slayer Magic can NOT eat God Slayer Magic, but the reverse can. It does seem that standard God Slayer is stronger though than base Dragon Slayer, unknown how the match-up would be against Dragon Force Dragon Slayer Magic
Based on this, I would say that Devil Slayer is strongest, followed by God Slayer, followed by Dragon Slayer (Base).
This is under the assumption that Devil Slayers use Curses as opposed to Magic.
However, if a Dragon Slayer were to be in Dragon Force, I would rank it as Dragon Slayer (Dragon Force), Devil Slayer, God Slayer.
So because of this form ability that Dragon Slayers are unique in having amongst Slayer Magic, I would say the final results would be:
1) Devil Slayer
2) Dragon Slayer
3) God Slayer