Where is the Most Fascinating Geology in the Solar System?

I've spent the last few days at the Geological Society of America Annual Meeting in Seattle. What I've learned is that geology has definitely left the planet and there are some great questions and locations for studying geologic problems on other planets in our solar system. This got me thinking about some of my favorite geologic sites in the solar system. Some of them are like features on Earth, others are like terrestrial deposits on steroids and others are like nothing we can find on our home planet. So, here is a top 10 of cool geologic features across the solar system (in no particular order). By no means is this exhaustive, but it is at least a taste of the amazing geology off the Earth. The seas of Titan:Cassini's visit to Saturn recently met its fiery demise, but one of the most important aspects of that mission was the Huygens lander on the giant moon Titan. There, the orbiter and lander took a close look at the remarkable methane seas on Titan. I'm fascinated with what might lurk in those seas under the thick atmosphere -- is there life? The ingredients of life? It might be awhile, but a robotic boat on the seas of Titan would be an amazing mission.