What do you want to use it for?
What is most important to you when you ask for “best?” Best price, best quality (although quality will depend largely on your goals as a trainer), easiest test, easiest to get CECs, access to more advanced programming, etc.
This has some good information about many certifications
Another one that’s oft-overlooked but excellent is PTA Global. I’ve attended lectures by many of their contributors and they’re top-notch therapists, trainers, physiologists and scientists.