The following are the basics that I think every gal should have in her wardrobe. These basics are the ones that make any outfit look put together so invest the time/ money to get them right.
Ubiquitous Denim
There should be at least 5 or 6 different pieces of denim in every wardrobe. I wouldn't be stranded on an island without at least one denim piece in my possession. My favorite denim picks are
1. Regular Denim Jeans
I own 3 - light wash, medium wash and dark wash. If you are planning to have more than one go for a different style for each (straight, relaxed, boot cut, skinny etc)

This is another absolute must for toning down very bright prints or being able to carry those in-trend pieces.

3. Denim skirt
Short and long length if you can afford 2. Short length if you can afford only one.

5. Denim Trouser
I wouldn't call this an absolute must but if you can, why not? :)
6. Skinny jeans
Go for cropped length like the Denim Minnie from J.Crew