Technically not an altcoin but I'm going to go ahead and nominate CRYPTOKITTIES to the poll.

1. First blockchain game on Ethereum
2. First ever DAPP that, while pushing ethereum's transaction scaling issues, is a great example of why ethereum is a real working platform for smart contracts and dapps
3. Appealing to a massive demographic that allow's newcomers get familiarized with crypto in a fun and friendly way