"Well, which certification is the best?"
My response usually includes something like;
"It depends on your goals," or, "make sure it's through a major organization such as NASM, NSCA, or ACSM."
Working in a large fitness center means there are quite a few other personal trainers on staff, all of whom are certified as a "personal trainer." What I've come to realize over the course of the past few years, is that most clients don't care about which certification you have, as long as you have one. What they really care about, or what will keep you as their trainer, is if you are a "CNP."
"CNP" = "Certified Nice Person"
People don't care if you have every certification in the world if you're not a "nice" person. And by "nice" I mean caring, personable, hard working, and genuinely interested in your clients. If I am ever in a position of management, I will hire a "CNP" with basic fitness knowledge over someone with tons of certifications, but little people skills, any day of the week.
Basically, what I'm trying to say is that...
people don't care how much you know until they know how much you care.