The worst insult I've ever received? God knows I've received a few but the worst one:

Average.

Calling someone average is worst than calling someone bad, it's like saying "Yo! dude, you aint even terrible enough to be classed as bad but you're not good enough to be good either." smurf you average!

Alternatively there's one I delivered before to someone else but it apparently elicits a most stern response:

*snip*

I snipped this for a good reason and this isn't even on topic. You are on thin ice, my friend. ~Shorty