As I tell my students who come to me with this question, a certification is only as good as the time and effort that went into obtaining it. And an instructor is only as good as the time, effort, and passion they put into every class or client’s program. If you really want to be a group fitness instructor, you will put in a lot of preparation and thought for your class design, choreography, and even music. I can always tell when an instructor wants to be leading a class. And a good instructor can pull that true desire up anytime they step in front of a group of people that came to have a great class.
That said, I am also partial to ACE for certification. After that it is up to you as to how good of an instructor you will be.