Is vitamin c the best cure-all vitamin?

No: There are no magic pills. The best "cure-all" has always been and remains eating a healthy diet and exercising regularly.

Somewhat: Vitamin c is a very important vitamin, but does not really cure any condition, other than vitamin c deficiency. High dose vitamin c via intravenous infusion has been shown in some studies to reduce sickness duration. All of the vitamins are important, as is a balanced diet. Many of the vitamins rely on one another. They do not generally operate completely independently.