Those popular coin which is bitcoin and ethereum is good to invest because they have a potential to increase in the future so lot of traders waiting for it. But i suggest bitcoin because bitcoin is the most unpredictable of all coins that have a big pump.
because Bitcoin is the most popular coin at the moment and also the highest transaction coin compared to altcoin so Bitcoin often gives a surprise especially if there is good news then Bitcoin can rise very high because big investors prefer to invest in Bitcoin.
and ETH is the best Altcoin with a very good project so ETH also has good prospects, but unfortunately that ETH depends on Bitcoin, and therefore Bitcoin is still the best.
Bitcoin not only popular coin in market but bitcoin is main currency in market. In exchanger, every pairs using bitcoin as main pairs and thats why bitcoin have big liquidity