Ethereum is the most established altcoin out there in the market so it's a good choice if you want to invest in it. Some of the other altcoins are based on ethereum smart contract so it just shows how effective ethereum is in the cryptoworld.
This is exactly the reason why Ethereum is such a good Altcoin. It not only beat it's ATH but it supports other altcoins using it's technology. It's so effective even to date.