No 2. Sobig.F

Infecting some 500,000 computers worldwide, Sobig.F caused billions of dollars in damage by stalling or completely crashing Internet gateways and email servers which resulted in the slowest of connectivities around the world. It was released in 2003 and it entered PCs through emails where it would gather email addresses and send itself to each. It sent over a million copies of itself in just a matter of hours after being released. Sobig.F is estimated to have cost over $35 billion in damages before finally deactivating in September of 2003, having being programmed to do so.

No 1. MyDoom

The daddy of them all, MyDoom, also known as W32.MyDoom@mm, Novarg, Mimail.R and Shimgapi, affected computers running Microsoft Windows. It was first observed in 2004 and became the fastest-spreading e-mail worm ever then. Like previous viruses this also spread through attachments and consequently forwarding itself to all the email addresses on the host computer. MyDoom caused a whopping $38 billion dollar in damages! It also slowed global Internet performance by 10 percent and Web load times by up to 50 percent during its reign.