Since you did not specify your question into altcoins, I would say Bitcoin is the fastest growing coin. Furthermore, since you did not specify what 'growing' in your question means, there should be no other answer to it but Bitcoin. In a matter of 10 years, Bitcoin has given us several thousands in percentage of growth in price. It started from 0 up to $20,000 in its ATH. From a digital currency used by no one, it has already millions of users all over the world right now. That is what growing means.