I think it's Etherum. In 2017 ETH is the most expensive altcoin, currently it has increased several times this year. I also hold a lot of Etherum. Hopefully in 2018 Etherum will continue to increase prices.
We almost have 30 days to finish this year. Here you can find my observations. Litecoin made almost 30x growth this year, cam here from $3 only. Another one is ethereum as you all know, dominated the whole year. The other one was bitcoin by making 10x-11x growth.