The best solid shampoos are non-saponified shampoos with a pH between 5 and 6. They contain the gentlest possible cleansing base so as not to attack your scalp, ideally sulphate-free. They are composed of ingredients of natural origin and certified organic that take care of your hair with all its problems. So pay attention to the percentages of naturalness and organic in their composition. Finally, ideally, they are made in France!
The best solid shampoos are also the ones that are right for you. The whole point is therefore to know your hair type well in order to choose the right shampoo! It is, for example, not because your hair gets greasy quickly that it is greasy!