The highest mountain on Earth is Mount Everest, and it's 29,029 feet tall. The deepest ocean canyon is Marina Trench and it's -36,069.5 feet. The difference is 65,098.5 feet.
Mount Olympus Mons is 82,021 feet. The deepest canyon on Mars is Valles Marineris, rather a system of canyons, and is 31,680 feet deep. The difference is 50,341 feet.
The greatest difference is the difference between Mt. Everest and Marina Trench. This difference is greater by 14,757.5 feet.