BTC is unique because it makes it easy for new wave of currency exchange, it blockchain has a unique wallet address, is not easily comprises to hackers or scammers, it benches on volume of communities members, it's God fatherism to other altcoins. Is transaction made easy and key or password individuals oriented. Bitcoin trades in a every sphere of market.
These things are minor and some not really true. Bitcoin is unique because it's the first decently-successful decentralized cryptocurrency that people can move around without needing anyone's permission but themselves.