They are the Federal Reserve System of the United States, Japan, China, and Germany, with the United States' assets being about 3 trillion more than second place - Japan.
Investopedia defines a central bank as follows: A central bank, reserve bank, or monetary authority is an institution that manages the currency and monetary policy of a state or formal monetary union, and oversees their commercial banking system. In contrast to a commercial bank, a central bank possesses a monopoly on increasing the monetary base.
The oldest central bank award goes to Sweden which was founded in 1668.