The best cert, is the one gyms in your area want.
(Fort Mill, SC)
I would investigate gyms in your city / town to see if there is a particular certification they are looking for. I am in the Southeast ( Charlotte, NC area). The flavor of the area here seems to be NASM so that's who I did my certification with. I have looked through the materials for other certifications and most seem very similar with a few differences. I have done some training down in the Huntsville, Alabama area and it seemed most trainers I chatted with there were predominately ACE certified. Once again I think it depends on what those doing the trainer hiring have had experience with in that area.
One certification that I haven't run into anywhere is ACTION. I actually took their test and downloaded the text as both were free. Once again similar to what I previously learned, the book is very well written and I will be keeping it for future reference. But they are not NCCA certified yet. I have not run into anyone working based on this certification. I believe this is due to its newness and the fact that most Of those in the position to hire trainers haven't heard of it.
All of this is strictly my opinion based on what I've observed.