Love every single one of those looks. Just going by the looks (and taking the event out of the equation), my favorite is 2003. She looked so simple but SO CHIC. Love that dress on her (though, the makeup looks questionable, maybe it's just a bad quality picture). I also really like 2008, though I think I may be in minority on that one. I remember people didn't care for it too much.
But when I take into consideration the fact that this is a MET Gala, I'm going with 2006. That was a bold choice, and she pulls it off amazingly well. I can't think of too many people who wouldn't be overwhelmed by that number.