the best out there are NSCA and ACSM, with ACE being 3rd.
ACE would be the best if just starting out without a decent background in exercise science.
WITS is an up-and-coming cert. that requires/offers 15 hrs of classroom lecture and 15 hrs of practical learning in a gym setting.
Then you have 20 hrs of internship.
This is nice, because you dont simply read and take a test and walk into an appointment with your first client blindfolded.
WITS stands for World Instructor Training School. Not sure the website, but google it and it should be the first one that pops up.
Hope this helps!
Wes