ACSM is by far the most respected, they are a medical certifier, they certify Orthos, PTs, etc. Personal Training is the lowest level cert they have, 10 years ago you had to have a college degree just to take their test and you had to administer a treadmill stress test as the practical, they have since dropped these requirements. The test is still very hard.
ACSM is good for three years also.