I don't know witch altcoins good to buy at 2018. Because about the condition of market. There's sometimes going up but suddenly going down. So if you are really need good altcoins foe holding, i can suggest eth,xmr. But i can't promise it.
Many question the best coins for 2018, as if they think I'm a magician or fortune teller, the question doesn't make sense.
Even they also have strong predictions, I think everyone is crazy because prices are getting lower, if they have experience about current market prices and current bitcoin prices, of course they will understand the current situation, I assume only people who have experience can understand the current situation.
My advice if they are looking for the best coins, they have to go to the market, because in the market they can choose what coins are often played by many people.