All of the afore mentioned certification program are excellent but I actually teach on the side for the World Instructors Training or W.I.T.S. out of Virginia Beach, VA. They are accredited as well and have a very nice comprehensive program that includes an internship program at a fitness club along with a practical and written test, plus CPR certification is a must. They have affiliations with many universities and colleges across the U.S. and I think they have a very strong program or I would not teach for them.
Also, having a degree in Health and PE, Exercise Science, Sports Medicine to name a few is also (in my opinion) tops a certification program because the student is will to get a BS or in some cases an AA in the field, putting in 2 to 4 years at a university rather than just a few weekends or on-line courses, hooah!