Perched above Gusev
I found the exploration of Gusev ad Endurance particularly fascinating, but if I had to pick only one image as my favorite, it might just be this one.
The foreground of the image is a wonderful jumble of Martian rocks, each and every one amazing in its own right, and the background conveys such an incredible expanse (I can almost see the towns cropping up already). And it's more than just pure distance...there's atmospheric haze!