One can always go with Ethereum, but should never go with one single coin only Other altcoins worth buying now are NEO and Litecoin. Cardano is very promising, too.
Agree. It's best to diversify your investment into other popular altcoins just to be safe. As such, Ethereum should be on the top of your list of altcoins to buy. However, Ethereum is not the only altcoin that would succeed as there are other competitors like Cardano, NEO, and EOS which are strong too. Doing your own research about a specific altcoin you're looking to buy, will allow you to feel confident that your money will be secure in the long term.
Sometimes, because of greed from certain altcoin projects, they tend to fail over time. However, altcoins with active development and a supportive community, will always remain on top no matter what. In conclusion, you should look investing into ETH, NEO, ADA, EOS, ETC, LSK, and ZEN. Just my opinion

Focussing on platforms for dApps is certainly a good strategy, all these will grow with applications that are built on their platform. There were a few missing to make the picture complete: RHOC, WAN, ICOS.