I think one of the best altcoin that I have now is PRIV this project about security and privacy. PRIV now is gaining more supporters and the price of this is gaining too. But I can't say that I will not sell it because I need to sell it to buy more when it goes down and sell it again when the price goes up. And you can't hold altcoin forever you should know how to let go. 
A lot of altcoins are having a great potential and with great performance all over the year, like ethereum it is one of the top altcoins that good for investment. but honestly i didn't believe 100% on altcoins maybe 85% in ethereum.