I'm still working on it, but I've made my own 5 star grading system for Devil Fruits. I've only really hammered out the first two stars, but I'll show you what I've typed up so far. I chose a star grade system because Devil Fruits are food technically. xD
1 star-
2 star-
3 star-
4 star-
5 star-
? star-
The criteria for distinguishing a star grade is that the relevant factors can only be based on that which is a resulting ability of the Devil Fruit(DF). Additional factors such as weapons, haki, martial arts, misc. are irrelevant to grading a DF ability, including cases where they were used in tangent with the ability(i.e. Mr. 5's explosive breath bullets; Luffy's Red Hawk, etc.; Rob Lucci's Shigan; Chopper's Rumble Balls), and yes that includes Zoan weapons.
The primary determining factors are strengths and weaknesses. The secondary determining factors are nature(this is to note strengths and weaknesses in relation to other DF abilities), variety(this is to focus on the strengths to weaknesses ratio and measure how thorough of gifts are given), and last is potential, or x-factor(this is to measure that which we are not seeing applied).
1 stars can be considered superhuman, but their abilities have a small number of features that aren't very dynamic. Users of this grade can easily overwhelm those without DF abilities of their own or remarkable strength or skill with their powers. This grade is predominantly Paramecias, but I would say Zoan weapons largely fall into this grade too.
2 stars are the most common DF grade for Paramecias and Zoans, but a rare grade for Logia. Paramecia 2 stars require they have an ability that has a wide range of potential applications, and they typically are an ability based on a theme. Zoans are naturally given enhanced senses, strength multiplication, and the use of both their own and their Zoan's physical tools in tangent. All notable, but what makes them all definitively 2 star grade is their potential to 'awaken'. Logia also are naturally at the very least 2 stars because of their resilience and maneuverability, and in this grade these qualities make up the dominant part of their usage.
I'm still working on 3-5. It's a complicated process because of so many factors. 3 stars aren't so far off from 4 stars. They are the hardest to determine. I know that predator Zoans and Ancient Zoans are at least 3 stars. Mythical Zoans are at least 4 stars or higher. I know I consider Luffy's to be 4 stars. Crocodile is tricky to place. I've got a lot of work to do with it. I'll probably try to establish 5 and 4 before I do 3 to make it much more distinguishable.