Hello Lori Warren,
Good for you on expanding your education. The best is a college degree: Registered Dietitian. If you want only a certification then there are a few choices. The best cert is what your future employer prefers and NCCA. Here is a list of many choices:
Natalie aka NAPS 2 B Fit.
If you have a Personal Trainer Certification through one of the bigger corporations most of the time they have an additional nutrition certification you can get for a reduced cost with your current certification.
If you don’t have one, I would look into this one: https://www.issaonline.edu/certification/nutrition-certification/. They are known to be very good!