In my opinion Ethereum is number one Altcoin in the world. There is no comparison of all other Altcoin with Ethereum. Second is litecoin because many people are investing in this currency. Third is ripple which is also a growing currency. Fourth is Dash and fifth is Monero. For me these are the top Altcoin of 2018. Different people will prefer different currencies because of their investment or they have observed the market.