This list of mine is heavily biased given the fact that I'm (even) more interested in Cassini-Huygens than e.g. MER:
* At least two images from Huygens, one during descent showing 'lakebeds', 'channels' etc. and then a post-landing image, preferably in color (the post-landing image is the image of the year IMO)
* The Enceladus plumes
* Hyperion hi-res global mosaic showing its weird appearance
* Images of Iapetus showing the equatorial ridge from the flyby a year ago - assuming these didn't appear as part of a collection of 2004 images. Also maybe images of Iapetus in Saturnshine
* The famous global image of Dione with Saturn and the ring shadows on its globe in the background.
* One of the images of Saturn's rings showing the opposition highlight (maybe this one:
http://ciclops.org/view.php?id=1187 )
* Maybe this one of Rhea behind Dione:
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=51392* Some of the beautiful Cassini images like this one:
http://ciclops.org/view.php?id=1112 (there are many more)
* An image of Itokawa showing Haybusa's shadow on the asteroid
* A hi-res global image of Itokawa
* A DI image showing the impact - the one where the bright plume is visible with its shadow also visible on the comet's surface
* Possibly a global image from Messenger's Earth flyby