I can't believe I'm saying this, but y'all can keep the Na'vi. My pick for the most impressive recent achievement in on-screen creature design is the Subsiders -- the gross, bat-like human-turned-vampire-turned abominations featured in the Spierig Brothers' fairly terrific dystopian fantasy Daybreakers.
This photo doesn't do the Subsiders justice, and their backstory (largely spoiler-free, if you haven't yet seen the film) is actually important to what makes them so profoundly creepy. The basic idea is this: In a world ruled by a civilized breed of vampire, humans are hunted and harvested for blood. Even so, the supply of human blood is running out, and shortages become widespread. If a vampire doesn't get enough human blood, he or she slowly begins to undergo a transformation from their essentially humanoid state to the primitive, brutish, barely sentient creature you now see before you. Predictably, the blood shortages affect the poorest strata of society first and worst, creating an underclass of Subsiders that the rest of the vampire world considers a menace.
Early in the film, our protagonist -- a vampire scientist (Ethan Hawke) searching for a substitute to human blood that may permit the human species to repopulate -- has a run-in with a Subsider, in his kitchen. With the exception of one sequence in Ti West's The House of the Devil, I found this scene more unsettling than anything I saw in 2009.
Unlike the Na'vi, obviously, the Subsiders are mostly practical, designed by make-up effects guru Steven Boyle. That's significant, but not really the key to what makes them so singularly creepy. What does it is that, uniquely among horror movie baddies, they are not supernaturally slick. Instead, they move like corrupted versions of human beings, which in fact is what they are. Their transformation gives them some measure of additional strength (and a pair of frayed-looking wings that let them leap onto ceilings), and they are perfectly capable of hurling you across the room, or taking a honking bite from your neck. At the same time, they're beaten down and desperate. Also ugly, in a way that is less fearsome than sad. At one point, a Subsider licks blood off a tiled wall, and I wanted nothing more than for someone to put it out of its misery.
With the recent proliferation of vampire movies, we've seen pretty much every variety of fanged beast, from the wussy sparkle-hunks of Twilight to the phantasmagoric monstrosities of the Blade series to the vicious predators of 30 Days of Night. But I don't think I've ever seen the combination of frightening and pathetic that makes the Subsiders bone-chilling. They are a triumph of both effects and storytelling. It's not just that they might eat you; it's that you might feel sorry for them while they do it.