Lots of vitamins are important, for different reasons. I don’t think that there’s one vitamin that’s more important than the others. However, different types of vitamin deficiency can cause different effects, some of which might be more serious than others (depending on how long the vitamin has been lacking). For example, a vitamin B12 deficiency can cause memory loss and irritability, and if left untreated can cause death in 3 or 4 years as a result of a certain type of anaemia. Vitamin A deficiency can cause blindness, and Vitamin D deficiencies can cause problems with your bones. So, they’re all important in their own ways, and we all need to make sure that we’re getting enough of each on a day-to-day basis.