NASM
Funny thing is, one of the most successful trainer I know ( makes about a quarter million a year training about 20 hours a week )
isn't certified ( and yes, he trains groups and charges about $45 per person for groups of anywhere from 4 - 12 people... too many in my opinion at one time but it works for him) No one EVER asks him if he's certified either.
I know plenty of other trainers with countless certs who barely have any clients and are almost always broke
I personally learned most of my training techniques from my own research. MM2000 (back when it had 2000 in the name), T-mag, Ian King, Poliquin, West Side Barbell, Michael Boyle, Siff and so on.
Also in the gym experience.
I think the certs give a trainer the confidence to say "I'm certified" and teach them some basics. But most of the real learning is best done outside of the curriculum
That said, if I were still training and were to go back and do it again I'd go NSCA, but I'm too lazy
Good luck,
Chris
KickBackLife.com