Hello Lisa Deere,
The one that is accepted by your future employer and/or NCCA accredited. Here is the link to check out some options:
Natalie aka NAPS 2 B Fit.
Natalie and Karin both have a point. Most of the places I have taught in the last decade want NCCA. But within that there are a number of options, and of those options ACSM was called the gold standard even back when I was certifying the first time in the early 90s. And ACE (with whom I certified at that time, and with whom I am certified now) is both well known and well regarded throughout the industry. And that is important. You might be working someplace now where they know you and just want something on record, but if you move having something really solid becomes more important.