Another must-have basic is the Breton top. It can be so easily dressed up and dressed down. I think Blaire Eadie at Atlantic-Pacific is a genius at doing this: with suits, with dressy skirts, dresses, casual skirts or boyfriend jeans, she always makes it look amazing.
When choosing one, make sure to get a simple cotton one long enough for a casual front tuck and loose enough to go over a shirt. Round necks are the most common, but you can decide if you want a crew neck or a lower scoop neck!
Be careful with the stripe too. It can be white with thin black stripes (my favorite), black with thin white stripes (a darker overall look), or bold black and white stripes (more graphic and generally more difficult to tone down).
If you can find a little extra detail, like a colored strip on the inside of the collar, or a dip on the bottom, that's always an added bonus!