I keep forgetting to check this corner of the forum. With the MER accomplishments, HiRise, Cassini, MGS observations of recent changes, and other wildly successful missions, this has been quite a fantastic year for space exploration. It will be quite difficult to choose the best few. I think you are going to need a large gallery.
The backlit Saturn images were amazing, but there were many awesome views from the bright side showing atmospheric banding overlain by rings and shadows of rings, sometimes with interspersed moons and their shadows that truly exhibited a fascinating view of projected geometry in space.
As for our beloved rovers, the view of the century had to be Spirit's long baseline stereo of the inner basin, but that was taken in late 2005. For Spirit 2006, various views of El Dorado were pretty nice, but I think the first image of Home Plate's cross section, when textures like the upper cross-laminations and the coarser lower layers became visible, would be appropriate. For Opportunity on the other side of the planet, an image showing the maize of drifts it miraculously struggled through to reach it's ultimate goal would be inspiring. I nominate one of Nirgal's colorizations. It would be difficult to leave a view of Victoria out of this collection, but which is best?
One last thought regarding the rovers...include something from the under appreciated MI. Along with the RAT, the MI turned out to be quite a useful tool. Don't forget the festoons.
Spirit returned the most diverse set of MI images. Between the two rovers, there were some pretty significant MIs that were sent home. It would be difficult to choose a favorite. Oh, and the dust devils...
I am so lost in the huge HiRise images that I only have a vague idea of my favorite crop. This has been quite a spectacular year for space exploration. I know that I haven't been able to keep up with it all.