As far as classics go, the LBD is right at the top of the list, along with denim jeans and nude pumps.
Like anything that you buy, these classics can be found at a wide range of prices so don't worry that you won't be able to find one that works for you!
If you're new to the LBD (and most likely, you aren't), it's basically a black dress that flatters your body shape (usually) and has a design that pairs with most occasions. Some people prefer a more fitting type of dress and others that may be body conscious prefer ones that are bit more flowy.
The fabric of the dress will also determine how versatile the dress is. Satin will give it a dressier and more formal vibe while cotton or polyester will probably tone it down, giving it a more informal look.
I would recommend finding one that is a solid color and not anything patterned. Something subtle would be okay if you're trying to find something a little more unique and suited to your tastes. An example would be a lace trim on the straps or maybe an interesting cut in the back.
Here are a few options if you're looking for one and you're on a budget!
And there you go! I really hope this post has helped :) Enjoy~