For the long-term investment, the coins that I think are the best-
1. Bitcoin (BTC)
2. Ethereum(ETH)
3. Stellar (XLM)
4. Waves
These are the stable coins. I hope these coins will grow very well in the future. In my opinion, there is no risk to investing Bitcoin and Ethereum at this moment.
I'm also pretty confident for your choice but I also discourage someone who would invest most of their money on stable coins like that because the chances to get bigger gains are somehow harder than buying some low cap coins. Maybe a fraction of your investment allocation just to diversify.
There are numerous projects with a high potential. For example, IOTA. IOTA is already working with many companies including Volkswagen, Bosch, Fujistu. I think that these co-operations will be decisive for a success. I can also recommend Ethereum, as 90% of the cryptocurrencies are already built on the Ethereum platform.
For ethereum, even though the creator itself Vitalik Buterin also doubts the potential of ethereum, it's undeniable that most of the crypto are revolving around it, it's one of the coins that have higher potential and proven it's utility on the market being the top2 on the marketcap.