Jessica Alba's ass
OK, so it's that scene near the end of the movie after her costume gets ripped, and Anton cuts her down and she kisses him. There is a shot of her ass in nothing but her underwear. Dear God, I paused at that moment and wept at the sight of what truly is the perfect ass. BTW, it's at about 1:22:54 in the movie.
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"Knightley is a poor man's Portman." -monkeypunks