Oh why do we have to pick just one? Actually my tastes have changed over the years I guess.
Back in college I would have picked "In Love and War" about Hemingway, with Sandra Bullock.
Mid 2000's I would have picked "The Hot Chick" with Rachel McAdams.
Then, Carey Mulligan. The Great Gatsby is the easy choice but she was better in "Never Let Me Go". I wouldn't say "hot", but she is naturally very pretty in a girl next door type of way.
Nowadays I have a thing for Kristen Bell. The Couple's Retreat and Forgetting Sarah Marshall are favorites. Quite honestly I even like listening to her on Frozen.