Where is the highest mountain?

It’s on Mars.

https://penningtonplanetarium.wordpress.com/2013/12/21/the-9-tallest-mountains-in-the-solar-system/comment-page-1/

The Giant volcano Mount Olympus is the highest mountain in the solar system.

It is almost 3 times the height of Mount Everest and so wide that it’s base would cover Arizona, and the whole of the area of the British Isles. The crater on the top is big enough to swallow London.

Mount Olympus doesn't conform to most people’s idea of a mountain. It is flat-topped-like a vast plateau in a sea drained of water--- and its sides aren't even steep. Their slight incline of between one and three degrees means you wouldn't even break sweat if you climbed it. 

Reference:

 Ask an Astronomer. (n.d.). Retrieved May 6, 2015, from http://coolcosmos.ipac.caltech.edu/ask/199-Where-is-the-highest-mountain-in-our-Solar-System-