I want to gain weight what are the best vitamin?

Diet and muscle mass: To gain weight you have to consume more calories than you are burning. Do this in a healthy way though. Don't choose junk or fast food. Keep a well balanced diet and make sure you are getting all the nutrients you need. Don't cut out exercise as it is very good for you. Weight lifting and other muscle building exercises can help you increase your body mass.

Lifestyle change: Health and the body you want don't come out of a bottle. Exercise and diet are the key for you -- stay adequately nourished; a cheap vitamin-and-mineral tablet will be sufficient, and stay fit so you develop muscle and solid tissue rather than excess fat.