There're a lot atm actually that will show their potential next year.

  1. OMG - this one is really safe going up and might probably be min 2-10x - big guys in the Team, great technology and also a lot of followers already

  2. Litecoin - Litecoin was always a safe investment and is a good choice for longterm although the price is a bit high right now. The technology and everything is great and they are
  already a long time in the game which will also keep them growing slowly all the time. So safe, but not the highest winpotential

  3. Lisk - great potentital with it's team and technology, but better read about this one by yourself

  4. Ethereum - safe longterm holding because it's all the time progressing and next year with Proof of Stake it could get a lot of gain.

  5. IOTA - read about, I think it could get really big and the price is still under 1$, so here's a chance to 10x-100x your money. Unfortunately there are just a few Exchanges offering it,
  but it might get a big rise when Bittrex and others are also offering it.

Actually these are the main ones I can recommend and might be safest options to invest when talking about longterm. Shortterm you can take a look at Vericoin as it always going up and down for around 50-80%.

Otherwise you should also take a look at those coins: Waves, Vertcoin, Ark, EOS, Golem.

Don't just go for one or two as you might miss big chances and splitting the risk is always a good thing.