ACSM Certified Personal Trainer is the "Gold Standard"
I'm a ACSM-CPT. ACSM is the standard for physical fitness. Practically all of the other certs (NSCA, ACE, NASM, NSCF etc.) reference ACSM material and publications. For example "ACSM Guidelines for Exercise Testing and Prescription" is the standard for testing and prescription in the industry. All of ACSM's training protocols are based on scientific evidence and years of evaluated results (Since 1954).
If you don't have a AA or BS degree in exercise science. I think ACSM is a great choice.
By the way, I did purchase the CPT training materials here at this site when I was studying for the exam. I found the material helpful as part of an overall study plan.