this is an often-asked question, and while there is no definitive answer, you cannot go wrong with an NCCA accredited certification from ACSM, NASM or ACE.
If you have a prospective employer in mind, you may check with them whether they have any preference.
Hello Nadia Rivera,
The best is always NCCA and what your future employer requires. Check around your area to see which cert is most often accepted. You may like to start here:
Natalie aka NAPS 2 B Fit.
You need to look each one in depth and see which one will serve your purposes the best. Each certification has its own strengths and weaknesses and also each one might help you train a certain type of clientele (depending what is it you are looking for). Also each one has various resources behind it that other don’t so you will need to check into those as well. Good luck you!