Well I don't know what the "best" anime of the decade was, but my favorite was without ANY doubt the Death Note series (the "relight's" included).
But most suprising was the incredible awesomeness of Afro Samurai and it's sequel-movie-thing. I would've never expected that concept to work that well. So those would be my picks for best anime, although to be fair Code Geass, Sword of the Stranger and Samurai Champloo and....okay, no point in doing that. Death Note and Afro Samurai it is.