the best coins to buy is always bitcoin  because bitcoin have a big chance to be increase more than now. but in altcoin, i am sure that there is many altcoin that we can buy and i think every coins is worth to buy and hold for a long term. but for me, i am still holding nem, waves, xrp, ltc, eth, xmr, and 5 more of coins. its depend on what you want to do with the coins, do you want to trade for short term or long term because if you want for short term, you don't have to wait for long time and you can directly sell when the price is increase. but if you want to hold for a long term, then no matter how much the price, you only buy and hold until some period and then you will decide in how much you want to sell.