Which planet in our Solar System is the coldest one?

Until it lost its planetary status in 2006, Pluto was the coldest with an estimated surface temperature between -235 and -210 degrees Celsius. But Pluto has now been relegated the status of a Dwarf Planet.

Uranus has an outer temperature of -224 degrees Celsius. It is colder than Neptune, as it lacks such a large moon as Neptune's Triton to heat it via tidal interactions. On Neptune, Triton's gravity warps the shape of the planet pulling "harder" on the face of Neptune closes to Triton (The farther away, the less gravitational influence), just like Earth's moon brings in the tides. This mechanically warping takes energy, draining the rotational kinetic energy of Triton, and ultimately converting it to heat. Uranus' largest moon Ariel is less than a tenth as massive as Triton, even though its a little bit closer to it's planet, and doesn't provide as significant tidal warming. This keeps Uranus colder, Even though Neptune is more than a billion miles farther away from the Sun, and gets less sunlight.

The temperature on the distant Kuiper Belt dwarf planets should be practically absolute zero, since they get virtually no solar or internal heating. Haumea, Makemake, and Eris are locked in a frozen state like Pluto.

Mercury Hot and Cold

Interestingly, since Mercury rotates so slowly, at night it cools down to -183 degrees Celsius. Not quite as cold as Uranus or Neptune, but very chilly! Since it reaches 630 degrees Celsius on the sunward side, even 88 Earth days in shade can't cool it completely.
Uranus is the coldest planet in the solar system.

Comment: That's just about right, but Neptune is more or less at the same temperature.
Uranus. But Neptune is very nearly as cold.