I think that fastest spreading still goes to SQL Slammer.
90% of possible infection covered in a very short time.
and I say that it changed the way that people work, before SQL slammer people were less clued up about firewalls, port 1433 was often just left open, and that's how the worms spread.
the patch to fix had been about for months, and not even MS had installed it.
after this people started putting up better firewalls, people developed patching strategies, (though conflicker outbreaks may suggest that the last lesson was soon forgotten)