I usually pick out beautiful actresses by their acting in a particular movie - Their beauty usually shines through in a role that suits them.
Nicole Kidman has lovely shifty, sparkling eyes and an intoxicating smile.
Jennifer Connelly has that dark, bewildered look I've come to associate her with. I find her very attractive.
Cristina Ricci in "The Ice Storm" - There's one particular scene when she's staring at Elijah Wood and it's a close up. She looks amazing.
Thora Birch is amazing in every role. She has a lovely smile.
Dominique Swain is just plain sexy.
That's all I can think of right now.