Technically, altcoins do not hold this much potential to serve you with long-time investments. Because its really rare that any lightening altcoin may come to give you this positive. Bitcoin is the only long-term investment for now, as people have a lot of trust in them and we are watching it for past 9 years.
If talking about altcoin, then Ethereum and Litecoin hold the potential to serve you long, but we are still unaware of the bums and dumps of the same. You can invest in other altcoins but it will get difficult to keep them for long, you will either exchange it to Bitcoin or buy some other coin through it in the end.