The Waters of Mars

Best.  This is hardly revolutionary, but The Waters of Mars is easily the best of the four specials broadcast in 2009.  In many ways, being the best of 2009 could end up damning Waters with faint praise, which is completely undeserved - partly because Planet of Death and End of Time (particularly) were schmaltzy pap, while The Next Doctor, while promising, features the uber-anachronistic Cyber-king rampaging through 19th Century London.

Waters benefits from Graham Harper's well-paced direction, a great cast, dialogue which is (for the most part) really tight, a claustrophobic atmosphere and the most terrifying adversary of the new series, bar none.  (Seriously, don't give me that Weeping Angels crap).  The Floods are implacable, calculating and atavistic - a microbial version of the Wirrn, who absorb knowledge as well as form.  Finally, for most of the story, the Doctor wants nothing more than to leave, something we've not seen since Frontios.