I go for the Ope Ope no Mi , "Operations fruit" I think , it's the strongest one , and it's the most hard to master, cos it has a limit time to use it and the aplication is hard as well.
But you are like God inside the space bubble , Logia fruits are useless in the New world , everybody got Haki and GG for logia fruits users ofc..
Example : the fight between Marco and Kizaru, Vergo and Smoker , etc...
1.-Operations fruit can't be contered by Blackbeard "Dark-Dark fruit", operations fruit , can be only used in combination with natural skills and natural objects.
2.-Operations fruit is able to has ∞ forms to use in theory.
3.-Operations fruit effect is somethings that you can't touch, feel or see , it's some kind of dimension when we would talk about physic.
4.-Of this way , it's practicaly impossible to conter it by 1 vs 1,only if the user mastered it hard . It would be enough , if Law would master it the double then actualy.
5.-Operations fruit can change and manipulate the target by ignoring the reality and physic rules , it's equal like some kind of God abilities, like Orihime Inoue by Bleach.
6.- It's obvious that Law can't dominate it perfect , in theory he would be able to do shambles with any particle in the air , he would teleport easily and be faster then light speed self , if he would dominate it better, finaly , he's the only one with that potential , when we talk about speed , speed haki or light speed can't be compared with teleport.
7.-If Law would got the same experience like whitebeard with his abilities , he would be maybe able to cut the whole earth in half...."he would destroy the earth with 1 hit"