I know you said only one, but how can you pick out of so many fabulous movies?
My favorite of all-time, and I'm not sure that it really qualifies as a "chick" flick, but Summer Lovers with Darryl Hannah and Peter Gallagher. It's old, but I could watch it over and over and over. The scenery is breathtaking and I just liked the story...a little quirky, but.....
I loved Overboard too, just saw part of it this last weekend; another classic. And of course, Pretty Woman.
OH...and While you Were Waiting...I LOVED that movie