Mercury

Mercury is the closest planet to the sun with an orbit of around 0.3 AU and it also has the most eccentric orbit in the solar system. It is very hot because of it being so close to the sun. Its axis tilt is almost 0, making the temperatures on the planet very similar in every area of the planet (compared to earth's 23 degrees which creates seasons).

Spacecrafts Visiting Mercury
The Mariner 10 spacecraft fly by mercury in 1974 and 1975 and MESSENGER which launched in 2004 and crashed into mercury in 2015.

Size
Mercury is the smallest planet in the solar system, but is almost as dense as Earth (Earth is the densest planet in solar system).