Which is the hottest planet in the Solar System?

Venus, at about 465 degrees Celsius all over the surface, day and night, poles included - hot enough to melt lead. Mercury is closer to the Sun and colder, because it has essentially no atmosphere and loses everything it gains as soon as the Sun goes down.

Venus has the opposite problem: an atmosphere of carbon dioxide about ninety times as heavy as ours, which lets sunlight in and will not let the heat back out. The surface pressure is what you would feel a kilometre down in the ocean. The clouds are sulphuric acid. Soviet landers reached the surface in the 1970s and 80s and returned photographs, and the longest any of them survived was just over two hours.

It is the closest thing we have to a control experiment on the greenhouse effect, and the reason it is studied so carefully. Venus is nearly Earth's twin in size and composition, may well have had oceans for a billion years or more, and ended up like this. It also turns backwards, and so slowly that its day is longer than its year.