Which group exercise certification is the best/most respected/most accepted at any gym?

While living in Peru in the last 2 years, I became certified to teach a style of cardio kickboxing developed there. I am back in the US and would like to teach kickboxing here but have found that I need an additional certification as a group exercise instructor. This is a little frustrating because I have my bachelors in kinesiology, my masters in athletic training, and my doctorate in physical therapy. I work full time as a physical therapist, and I just want to teach once or twice a week because the class is really fun and I want to share it with others. I am willing to get the certification but because of my background and the cost, I want to know which is the BEST cert for group exercise instructors. I want to be able to do this once, learn something, and be able to use it at any gym. Thanks!!