It is difficult to say with certainty which is the best personal trainer certification program because hardly anybody goes through all of them. If a person gets certified through the American College of Sports Medicine (ACSM), for example, he or she most likely won't have first-hand knowledge of other certification organizations, so any comparison will be somewhat incomplete. Furthermore, people can have very different opinions on the same certifications based on different experiences and goals. That said, it still can be useful to take a look at recommendations from people, and thanks to the internet, we can easily see what people are saying.
Keep in mind that certification programs can change, so what might be the best personal trainer certification today, can be something else tomorrow. So it is a good idea to confirm for yourself what is stated below. In looking at different sources from around the web like forums and answer sites, here are some general thoughts about personal training certifications:
ACE (American Council on Exercise) can be a good choice for someone just looking to get certified; it provides traditional knowledge, and most people should be able to pass the exam.
NSCA (National Strength and Conditioning Association) and ACSM (American College of Sports Medicine) are well-respected. The NSCA offers a Certified Personal Trainer (CPT) and Certified Strength and Conditioning Specialist (CSCS); the CSCS is more difficult to obtain and can be helpful if you want to train more advanced athletes.
NASM (National Academy of Sports Medicine) certifications also are widely accepted.
ISSA (International Sports Sciences Association) is mentioned frequently but is not accredited by the NCCA. It still can be a viable certification.
There are plenty of other certifying bodies that will help you land a job. There also are personal trainer schools that have courses based on different certification programs; these schools should be looked at individually because there are so many of them, and they have different pros and cons.
It can be useful to look at job descriptions to see which personal training certifications are accepted in specific work environments.
While you do want to make an informed decision before you begin a personal trainer certification program, keep in mind that there are more important aspects to a career in personal training than where you get certified. You will need a certification to get your foot in the door, but then it will be your ability to get clients results that will impact whether they retain you and recommend you to others.
And don't forget, the best personal trainer certification for one person might be completely inappropriate for somebody else. You need to choose the program that is best for you based on cost, convenience, your current knowledge, where you want to work, and other factors. Learn what you can and then make a decision so that you can move forward with your personal trainer career.