this year the best altcoin in my opinion is Ethereum, NEO, and Litecoin
these three coins have a big community in the market so I believe the future is very bright and suitable for investment.
Growth wise these all coins went to cheap rate but I do not know why you have suggest these coins for best altcoins for this year. For all time best you may choose ethereum at anytime but from January first to now. It goes more than 300 percentage dump in the marketplace.
If you want to see the coin still bumped after it reaches to the market means you may choose the Vechain and EOS. Simply rocking in the dump time as well.