Aagh! What a tough call. W has really had a superior set of covers this year, at least by comparison.
I really (and I do mean, really) love Brad Pitt and Lara Stone's respective covers, Lara's in particular; beautiful! I was also endlessly pleasantly surprised to see Demi Moore and Ginnifer Goodwin, neither of which have been overexposed in recent media imo, featured on their respective covers. Demi in particular, is such a stunning woman, and I dare say her strong cover does her justice.
My vote, however, goes to Linda Evangelista's extraordinary November cover. You can't really fully appreciate it until you've seen it live; I feel that the colors wash out a little digitally. The idea is brilliant, the colors and settings are fantastic, and tremendous props to the magazine for minimal Photoshop activity here. I don't even like Linda E., but when I first saw this cover I grabbed it without even opening the magazine.