Depends on what you want to do. ACE or NASM are good basic certs. NSCA IF YOU WANT MORE sports conditioning. ACSM is a winner. Being a PT and a former Regional Fitness Director for a large Health Club chain, I recommend checking out PTA Global. They have the most common sense approach and practical learning that is immediately applicable. NESTA and NCCPT are still NCCA accredited but are also less expensive.
Consider where you want to work and who you want to serve and how you want to serve them. If you have a mentor or role model, ask. Do some research into clubs and studios that offer the standard of service you want to be branded or known for and ask about what they require or like to see in applicants. That will get you started.
At our club we preference ACSM, NASM, NSCA and ACE – preferably beginning with PT and going on to the Advanced cert. More than likely if you continue in the industry within three years most trainers I’ve worked with have two certs under their belt; so don’t feel you can make a “wrong” decision. You’ll just be making a first choice!
The certification isn’t necessarily your learning experience; it’s your proof of the knowledge you have and then you’ll be learning practical ways to apply in with continuing ed workshops and conferences, articles, webinars, and experience.