I know the thread is quite old, but which coin to buy changes all the time. At the moment I would say forget alts and go with BTC... it has started the charge to $6000, and if it breaks through I think there is a good chance that the general market uptrend of the last couple of months can turn into a full bull run, with BTC leading the way.
in any case, it is Bitcoin and ethereum that are today the most relevant investments that will bring very good returns. In any case, in early 2019, the cryptocurrency market is already showing good results.