I regularly get "you are the worse mum in the world". My response is "glad to hear it, means I'm doing my job properly".
When my now 13 year old was 3, she got all flustered and called me the worst thing she could think of "idiot f@rking head". It has stuck around in our family, probably due to my reaction at the time. Most people laugh if they hear it, but it really rattled me she was only 3.