I had a pretty bad one at one time it was a Trojan.Zlob called "Security Toolbar 7.1" It hyjacked my homepage, slowed down my internet, smothered me with pop ups wanting me to buy rouge anti-spyware, it even made my antivirus not work. Eventually we had to call Symantec to remove it which cost us $100
.Here is a description of the virus: (Article website contained spyware threats, sorry. Just search for Security Toolbar 7.1 removal if you want to know more about it)
If you had a worse virus, post a description of it, and lets find out who had the worst virus ever!