Neptune. It is similar in appearance to the Great Red Spot on Jupiter. There are several other dark spots on Neptune as well as the larger one, but the Great Dark Spot is a name often given to all of them collectively.
Neptune's Great Dark Spot is a similar size to Earth, measuring around 13,000 kilometers in diameter. Around it, winds were measured with speeds of up to 2,400 kilometers an hour, the fastest in the Solar System.