If by Hax you mean versatility it's Gomu gomu no mi
If you mean biggest unfair boost it's Enma
Gomu gomu no mi was is trash without creativity whereas Enma granted to Zoro the most overpowered PU without any training
Creativity? You make it sound like it's hard to stretch your body to attack.
Gomu gumo gives you a god DF power and new ability you don't have before. It gives the user more energy, brought them back to life, rubber body, allows him to take attacks with less damage and more.
Enma doesn't give the user any. It forces the user to release more haki if that can't control it.
That could be huge burden to the user. It held Zoro back most of the war.
Having enma is similar to training with 50 lb weighted vest. It makes everything much harder at first but in time it can help you grow stronger.