Really, for long-term investments the bitcoin is still the main investment direction, but, certainly, there are some alternatives which as I think, are good for investments, such as Eth, Iota and Stratis, perhaps, in the next 1 or 2 years, certainly, will benefit from these investments.
Ethereum, monero, litecoin are good options for long term altcoin holding. Within 6-18 months, we can witness a huge price inflation in these altcoins honestly. I predict this for the last weeks.