I know certain fitness gyms are beginning to go with NASM for their personal training staff. Personally, I don't think you can go wrong with NASM, ACSM, or NSCA, though they are certainly the more difficult to prep for without some sort of background to begin with. ACE is certainly a good starter, but have kind of fallen out of the upper eschelon over the past 10 years.
In my time studying Kines in college, I'd say that NASM, NSCA, and ACSM are probably the more respected right now. NASM has been geared more towards functional training in recent years. NSCA has both Personal Fitness Training and Certified Strength and Conditioning Specialist certifications, with the latter geared towards athletics, particularly team sports. ACSM is probably the most reveared in their research, with a good deal of their focus pertaining to special populations and the everlasting fight against fat people.