It's hard to think of "next" because the most prolific crypto-currency is still the first one. Having said that, look at the ones that businesses are pouring money into...BTC, ETH, FileCoin. For BTC and ETH, you could follow the buzz for a while but now you have to be leery of market manipulation. If you play them, play for the long game. FileCoin is new but had a massive fundraising round. Expect it to go places.