So then, why not last year? You had, arguably, Tarantino's best film, a real Coen masterpiece (although not their only one) in A Serious Man, Drag Me to Hell, Antichrist, The Hurt Locker and Avatar...
In 2007 you had Ratatouille, No Country, There Will Be Blood and Zodiac. Does The Assassination of Jesse James count or is Dominick's filmography too small?
In 2006 you had Children of Men, United 93, Letters From Iwo Jima and Casino Royale (you best believe it's Martin Campbell's masterpiece).
In 2001 Battle Royale, Moulin Rouge, The Royale Tennanbaums, Mulholland Dr., Y Tu Mama Tambien, The Devil's Backbone (I would argue, likely futilely, that it's Del Toro's best), Fellowship of the Ring and Spirited Away...
My point is, landmark years for directors are a dime a dozen. One could argue that any one of the movies I just listed represents that director's best work, or most complete expression as an artist... though I'm still not quite sure what we're talking about.