I know that it's probably cliched, but I think Sharon Stone is rather sexy. Looks AND brains!
I saw "Phantom Menace" for the first time recently, and found myself having unhealthy thoughts about Natalie Portman.
Jessica Lange has always been a favourite. Especially in "Tootsie".
Having said all of that, you can't beat Winona Ryder. OK, so she's appeared in a lot of dreck, but she's far and away the sexiest film star.