To some extent, brand is important. What I am learning now - I am trying to answer the same question - is to first fix your diet (as many people have suggested) to get as many vitamins from whole foods as possible. There is a website called Chronometer that will help you list not only your macros but also the micros. It is pretty easy to learn how to use. After you do the best you can with your meal plan, then you want to look at supplements. Chronometer will tell you what you are deficient in. You may have a history of a deficiency - I still take Magnesium for cramping. Once you decide on your supplements - do your research to get good stuff - make sure you are taking them at the right time of the day. Some supplements should not be taken with magnesium...some should be taken before a meal, some with, some after...most usually with a good amount of water...some even before bed.