the bee's knees (idea)

'The bee's knees' is an expression which denotes that something is desirable or the best of its kind. It originated in America in the 1920s along with a number of other two-word expressions which were constructed in similar animal/anatomy format, although there were exceptions to this format.

Some further examples :

Assorted animals :

• The elephant's adenoids
• The bullfrog's beard
• The gnat's elbows
• The monkey's eyebrows!

Felines :

• The cat's whiskers
• The cat's miaow
• The cat's pyjamas

While 'the bee's knees' is certainly slang and does indeed rhyme, it is not actually rhyming slang in the normal sense, since true rhyming slang consists of a phrase which rhymes with the word that it is substituted for, such as 'butcher's hook' for 'look'.

Source:
http://www.worldwidewords.org/qa/qa-bee1.htm