Obviously , bitcoin should be the best altcoin to invest for 2018. As a matter of fact, bitcoin have a good value and it keeps increasing. This 2018,  bitcoin are still  the best to invest.Bitcoin is good for short and long term investment that's why don't  be fooled in other altcoin.
I think Ethereum should be the next from bitcoin then followed by Ripple. These three altcoins are the best altcoin to invest for 2018.