Here are some Alt coins you can buy in 2018 :

ETH: It can give returns better than BTC.

Ripple : Ripple is available for quite cheap.

EOS   : It gave huge  returns recently 2 months back.

Litecoin : It is popular because of less fees.

You can also buy other coins like Tron, ETh classic , Monero, Steem, Bitcoin cash and BAT.