Here are the tallest mountains on terrestrial planets:
Mercury - Caloris Montes - 5.5 km high
Earth - Mount Everest - 8.848 km high
Venus - Maxwell Montes - 20.7 km high
Mars - Olympus Mons - 27 km high
Note that the heights listed for these mountains are relative to their surrounding terrain. For example, Mount Everest's height is measured from sea level, while the other mountains are measured from the average elevation of their planet's surface.
