I actually find it hard to insult people deliberately, though I do it accidentally all the time, such as saying Tarks looks like the ginger bloke in Mythbusters.
One of the best accidental insults aimed at me was from a co-worker in Manchester who started off by saying that my girlfriend (now wife) looked like me.
That was fine, because many couples do resemble each other. I've been mistaken for one of my wife's brothers in the past. Thankfully it was the good looking one.
However, he followed that up by saying "I don't mean your wife looks like a bloke. She's not ugly",
and then "Or you. You're not ugly either."
And it all went downhill from there.