Mine's got to be "Peter the spastic paedo" at the moment. The very idea of a spasticated paedophile has me giggling like a schoolgirl every time I see it. What's yours?
David Nalbandians Rancid Dungbox was one of my favourites. Peter the spastic paedo is the best username ever, and very apt for this forum as it represents 82% of the users.
I don't really have one at the moment. I quite fancy changing my name to Moral Turpitude at some point in the future, but I've used up all my name changes.