ACSM (can also be ACSM-HFS or ACSM-PT, or both) is certainly a good one to have, along with/or CSCS, NSCA-CPT (there are also other good ones for kettle-bell training and other specialties). I agree with the caution against ACE and AFAA certification, sadly) That isn't saying that someone that only has that is bad - by no means - but in this case I would certainly seek out recommendations from current or former clients.