The hottest planet is Venus. The surface temperature of Venus is hotter than 460 degrees Celsius, which is 860 degrees Fahrenheit. That is hot enough
to melt lead!
Even though Venus is further away from the sun than Mercury, Venus is hotter than Mercury. This is because Venus has a thick atmosphere
and Mercury has none, so Venus has a greenhouse gas effect which traps the heat of the sun on its surface. Meanwhile, heat just bounces off
Mercury. Although the parts of Mercury that are in direct sunlight can get almost as hot as Venus-- 430 degrees Celsius-- the parts of Mercury
that the sun is not shining on can be as cold as -160 degrees Celsius, even colder than Jupiter or Saturn. Whereas Venus is hot all over.
So Venus is definitely the hottest planet, both by its highest possible temperature, and also by its average temperature.