In my own opinion and for me the best coin to invest for a long-term is Bitcoin. You can profit a lot when you buy bitcoins because it has potential to boom in the next years which many investors expect and predict. Don't invest in new coins when you are investing for the long term because these new coins are the most likely to die out sooner.
I would add that not only is bitcoin very interesting for long-term investments, ethereum is also a reliable coin, suitable for long-term investments. It even seems to me that ethereum looks more promising in terms of modern technologies.