Ethereum is far and away the foremost DApp platform as of April 2018, as well as being the second largest blockchain by market cap, having a value of $63.4 billion as of April 25, 2018, which represents roughly 18% of the total cryptocurrency market.
Ethereum was proposed and founded by Vitalik Buterin in 2013 and development was funded by a crowdsale that took place the following year, with 11.9 million Ethereum coins pre-mined for the crowdsale. The system finally went live on July 30, 2015.
The Ethereum network allows for open-source distributed computing on the blockchain, using smart contract functionality and a Turing-complete virtual machine called the Ethereum Virtual Machine (EVM). There is a wide variety of DApps that can be built utilizing the Ethereum platform, and already these range from social media DApps to gaming DApps, to prediction DApps, and much more.