There's no single "lightest and strongest" material, as strength and lightness are often competing properties. However, here are some contenders for the title, depending on what you mean by "strongest":

For Strength-to-Weight Ratio:

* Graphene: This one-atom-thick sheet of carbon is incredibly strong for its weight, with a tensile strength roughly 100 times that of steel. It's also incredibly light and flexible. However, it's currently difficult to produce in large quantities.

* Carbon Nanotubes: Similar to graphene, these cylindrical structures of carbon atoms have incredible strength and are very lightweight. They're being explored for use in everything from aerospace to medicine.

* Aerographene: A newer material made by creating a three-dimensional network of graphene, Aerographene is incredibly light and strong. It's been touted for applications like energy storage and filtration.

* Spider Silk: Nature's masterpiece! Spider silk is remarkably strong and lightweight, with a strength-to-weight ratio surpassing steel. Researchers are trying to replicate its properties for various applications.

For Sheer Strength:

* Diamond: Though heavier than some of the other contenders, diamonds are incredibly hard and strong, making them resistant to scratching, breaking, and other forms of damage.

Important Note: The "lightest" material in the world is likely to be a vacuum. It has zero mass, and therefore, is "lightest."

Choosing the "Lightest and Strongest" depends on:

* Specific application: What are you trying to build or achieve?

* Cost: Some of these materials are incredibly expensive to produce.

* Scalability: Can you produce enough of the material to meet your needs?

Ultimately, the best material for any given application will depend on a combination of factors, including strength, weight, cost, and availability.