Love the line in Aliens when a chauvinistic marine asks a female marine " Rodriguez, have you ever been mistaken for a man ? " She replies " No , have you ? "
My own personal fave is " I've stepped in sh'it that's harder than you"
I once got into a scuffle with a lad and used the 'I've dropped bigger turds than you' line. It was just before he actually got in first and smacked me lights out.