You're the best = You're the cat's pajamas. 
 
"Cat’s pajamas — Something considered to be outstanding.
 
The term ‘cat’s pajamas’ comes from E.B. Katz, an English tailor of the late 1700’s and early 1800’s, who made the finest silk pajamas for royalty and other wealth patrons. Nothing like a cat nap in Kat’z pjs. (from the book, ‘Cats out of the Bag’ compiled by Terry, Don and Ken Beck)"