Neptune is the coldest planet in the Solar System, maintaining a fairly consistent average of -200 degrees Celsius. Towards the planet's equator, temperatures can get as cold as -218 degrees Celsius, while temperature at the South Pole is about 10 degrees Celsius warmer.
The discrepancy in temperature has to do with the fact that Neptune's South Pole is exposed to the sun. Since a year on Neptune is equal to 165 Earth years, the South Pole is the warmest point on the planet for about 75 years at a time. After this, Neptune advances far enough in its orbit that the South Pole no longer receives the most sun exposure.