I can't think of any celebrities who even qualify as sexy. Maybe Tyra Banks--at least she's got some curves to her.
The sexiest woman on the planet, bar none is... I dunno. Nigella Lawson heads the list. There's a few others up there. That chick that played Hugh Grant's love interest in Love Actually was pretty damned hot. If she doesn't have wide hips and large breasts and a round bottom and a well proportioned figure AND MOST IMPORTANTLY a beautiful face, she doesn't qualify as 'sexy'.
I'll give Kiera Knightly a pass. She's reasonably hot, for a twig.