A Convertible Bra
What it does: Of all the bras you don't have, it's likely you've avoided purchasing this one. It's definitely the most complicated one to figure out, but that's because it's the most versatile. Its modifiable straps can transform into a halter, racer-back, one-shoulder, or strapless style. Owning one of these keeps you from having to own a handful of other rarely used bras.
When to wear it: Whenever you need a quick solution for a complicated outfit without the time or cash to buy a specially made bra.
When to replace it: As versatile as these are, you likely aren't wearing one-shoulder cocktail dresses too often, so consider this backup-plan bra one you can keep in the back of your closet for years before upgrading.