I'm surprised no one has mentioned the origin of "bee's knees". It is a slang homophone for "business" started around the Roaring '20s. Saying something was the business meant it was the real deal and of high quality. See business definition #4, #9, #12, etc.: "top quality. a perfect 10. met to standard."
Dude, that kickflip you just did was bee's knees.
Dude, that kickflip you just did was business.