In my opinion, the best indicator as to whether a cryprocurrency has a long term future is related uptake and how much they are actually being used and applied to practical purposes. At the moment, I would rate the three leaders in this area as Bitcoin, Ether and Litecoin.
Outside of these three, the one I like is Decred (pronounced Dee-cred, as in, short for decentralized credit). To me, the structure, development and principles behind it seem nicely streamlined and make more sense than many of the newer coins that are out there. The official website for it is Decred.org. It launched in February 2016 and was created by some former Bitcoin developers in an attempt to overcome some of the problems that they saw with Bitcoin.
Below is a chart of Decred's price from its inception to the time of writing. I think there is plenty of upside from this point and that it could well be the next big thing in crypto. Then again, the cryptocurrency market is highly volatile and difficult to predict, so I could be wrong. Time will tell.
I'm interested in seeing who emerges on top with Decred, Dash, Pivx, Zcash and others. Some interesting differences between them all
Thanks for reading and commenting @davidwallace. Interesting times ahead for sure. With all of the differences, I think uptake and usage will determine which coins are successful and thus have a sustainable future.