It really depends on what area you want to go into.
ACSM would be best if you're looking to go into the academic side of training (teaching classes at the college level).
NSCA would be best if you want to get into training teams of athletes.
ACE is a good choice for a personal trainer.
AFAA is a good choice for a fitness instructor.
Don't know too much about ISSA, have heard of it, and know it's fairly well respected and accepted, but once again don't know any details.
In the end though, just realize that being certified doesn't give you the ability to teach what you know. It takes practice, experience, and in my opinion some innate ability to become a really good teacher (whether it be coach, professor, trainer, etc...).
Certifications only get your foot in the door. From there you must continue to educate yourself (both on exercise methods and teaching methods) if you wish to really develop your skills as a coach/professor/trainer/instructor.
Also realize that most gyms don't care who you are certified through, only that you are certified.