Name the coin that has the highest market capitalization for all transactions in 2018.
highest market capitalization refers to max(market_cap); in 2018 refers to year(date) = 2018
SELECT T1.name FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE T2.date LIKE '2018%' AND T2.market_cap = ( SELECT MAX(market_cap) FROM historical WHERE STRFTIME('%Y', date) = '2018' )