Short list of my top 3 coins that will see the biggest gains by 2020:
1. Matrix AI Network
2. NEO
3. EOS
About NEO I would not be so optimistic, I do not even know what the situation will be in 2019. I think that for such a long time you can only trust bitcoin or ethereum.
Ethereum could easily be surpassed by a competitor by 2020. I don't think it is unwise to invest in some of those competitors (like NEO and EOS), but you should probably also hold a bit of ETH. (just in case
)