Hi Gary. "Top," or "best" certification is somewhat like answering the question "what is beauty." :-) The answer, "it depends on who you ask" (as I'm sure you will find out when the answers to your question start filtering in. A lot will depend on the focus of your business. One of the certifications that I hold is the CSCS from NSCA which personally I think is a great certification to have (particularly if your practice will involve work with athletes). The CSCS tends to focus more toward the athlete clients, so if that's the market that you're looking for, it's a good one. Like many other certified trainers, I also hold several other certifications from other organizations, but again, I've chosen the ones that I've chosen based on my personal interest, the
areas that I want to focus my practice in, and sometimes even cost.
In addition to soliciting answers on this forum, I also recommend that you check around in your local area to see what certifications other trainers there have. What I sometimes find is that most popular and recognized ("top?") certifications can sometimes be locale based, and supported based on where you are. This may be less of a consideration for you if in fact you are starting your own practice.
The bottom line is "top" or "best" may come down to how YOU feel about what your research tells you about the course, the cost, the prerequisites to sit for the certification exam, the types of certifications that the organization offers etc.
Good luck, and remember that in the end, the decision is YOURS and yours alone.