If we are talking of the fastest growing coin, then it is obviously going to be bitcoin, bitcoin is still the largest of them all with a very strong community that is very good at marketing the coin to the world, so there is more awareness for bitcoin than every other coin you see in the market.

But, if you are talking of the fastest groin altcoin, I would have said that it is Ethereum, Ethereum use to be the hotcake of all altcoins in the past, but I have really not known what the problem of the coin is of recent, it has really not been moving and the hype is slowly dying down seriously, while I think ripple is gradually taking over with the way they are marketing their coin and now showing the world at large how useful they are.