Bitcoin
Also called: BTC, digital gold
Bitcoin is the first cryptocurrency, launched in 2009, with a fixed maximum supply of 21 million coins and no issuing authority. Its design solved double-spending without a central ledger keeper, which is the achievement everything since builds on. Whether it functions as money is contested — it's too volatile for payments and too young for the store-of-value case to be settled — but it is unambiguously the most established digital asset.