Here are some of my favourite insults I have come across:

"He would fail a brain scan"
"If you spoke your mind you would be speechless"
"Does your IQ reach double digits ?"
"Has anybody ever told you that you are incredibly average?"
"Well I would agree with you but then we'd both be wrong"
"You're as bright as a black hole, and twice as dense."
"I don't care what people say about you. You're alright."
"So you're the reason we have warning labels on everything."
"If you were given an enema you could be buried in a matchbox."
"I don't have the time or the crayons to explain this to you"
"Your birth certificate is an apology from the condom factory."
"Your the load your mother should've swallowed."
"You have delusions of adequacy."
"The better half of you dribbled down your mom's chin."
"I thought life was a bitch. And then I met you."
"Two wrongs don't make a right, take your parents for example!"
"You ain't half the man your mother is."
"If your relying on your wits, then your halfway there"
"I won't engage you in a battle of wits, your unarmed"