I've just posted my annual roundup of significant images from planetary exploration in 2010. I make no claims for these being the "best of 2010" -- the images are selected more to mark significant events in 2010, though I confess some are just chosen because they're spectacular. And anyway, this is space exploration; I think all the images are spectacular.
NASA / JPL-Caltech / SSI / color composite by Gordan Ugarkovic
Enormous storm on Saturn
On 24 December 2010, Cassini captured its first complete photos of a massive storm on Saturn that had already captured the fascination of amateur astronomers. The storm was first noticed on December 5 by the Radio and Plasma Wave Science (RPWS) team on Cassini, who detected its lightning emissions and alerted astronomers. Amateur astronomers photographed it shortly after.