Most times it is difficult to decide. But personally I have chosen ETH, XLM, XRP and NEO to add to BTC as my investments and I am confident that those will give me something reasonable within the next few months. But there are other altcoins that are not yet in CMC but doing wonders too. But they are too dicey though.
Totally agree with you. Because I have a similar set of altcoins in my portfolio. But I would also advise buying Litecoin and EOS. Very promising coins.