Is there an American version of “You really take the biscuit!”? As in taking the last biscuit, i.e. it's incredible how selfish you are.
Take the biscuit or cake is excelling in something - positive or negative
So on its own it means nothing; after some act or some expression, it means you surpassed whatever was expected.
Here is my take
When it comes to selfishness, you really take the biscuit/cake
That was amazingly kind of you, you really take the biscuit
http://www.worldwidewords.org/qa/qa-tak3.htm has a rundown of etymology.