(something) is #1
The best, greatest, most important thing in some category is "#1" (pronounced "number one").
You can use this phrase to talk about things like
- an athlete or sports team
- a company in an idustry
- a product
If you want to specify what the category is, you can say "#1 in ___":
They've been #1 in sales for the last 20 years or more.