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"
"If I wanted to kill myself I'd climb your ego and jump to your IQ"
"I'd like to see things from your point of view but I can't seem 
to get my head that far up my ass"
"If you are going to be two faced, at least make one of them pretty"
"I bet your brain feels as good as new, seeing that you never use it."
"If you're gonna be a smartass, first you have to be smart. 
Otherwise you're just an ass."
"If I wanted to hear from an asshole, I'd fart."
"Some drink from the fountain of knowledge; you drink from the toilet bowl"
"Learn from your parents' mistakes - use birth control!"
"Aww, it's so cute when you try to talk about things you don't understand."
"Ordinarily people live and learn. You just live."
"I heard you took an IQ test and they said you're results were negative."
"You are the worlds first surviving brain donor"
"You are the worlds first surviving heart donor"