In order to choose one of the most effective and powerful Devi Mantra, you will have to list down your requirements. What do you want?
Do you want your children to study or are you studying yourself? Do you want money? Do you want power?
Do you want fame, and then my dear friend there is only one simple and easy Mantra which suffices all your requirements?
Here is All-in-one Mantra which is capable to fulfill all your heart desires. Only you need to change your japa form for different wishes.
Here is Mantra-
Similarly you can change your form of Japa with your requirement. Such as if you want education or knowledge you can chant
In the same way for money
As mentioned above, my friend I can call this Mantra an all in one solution Mantra as one can change the requirement and see his/ her faith in goddess in the form of what they want from her.
Similarly you can change words such as kshudha (hunger), lajja (chastity) before Reuben (रूपेण) and chant mantra to get the desired benefits.