Which planet has the tallest volcano in the solar system?

Olympus Mons is a shield volcano, built up by lava flows over a very long time, like Hawaii's volcanoes but much bigger. It stands about 22 km above the surrounding average Martian surface and is around 600 km wide, similar in size to the US state of Arizona. At its summit is a caldera about 80 km across, made of several overlapping collapse craters. Volcanoes on Mars can grow so large because the planet has lower gravity and no moving tectonic plates, so a volcano stays over the same hot spot for millions of years. Its slopes are so gentle that someone standing on them would barely notice they were climbing a mountain.