I think it's ETH. Looking at the trading volume, you can see that. But I think ETH is only for mediocre traders, meaning they don't have big assets. For whales, they will only trade with BTC and want to hold it more.
For me, the most popular altcoin is XRP and Nano. Nano supports us for transaction fees and we don't have to charge, but it appears to be a lot less exchange. XRP is better with extremely cheap fees.
I don't have too much money so I often choose such altcoins for trading and investment.