The Top Ten
1. ILOVEYOU - (2000) One of the most widespread and rapidly spreading viruses ever, the ILOVEYOU virus spread via e-mail, posing as an executable attachment sent by a friend from the target's contact list.
2. Code Red - (2001) IIS on Windows servers were the target of this virus. It also launched denial of service (DoS) attacks.
3. Nimda - (2001) Nimda used seemingly every possible method to spread, and was very effective at doing so. Nimda is notable for being one of the fastest spreading and most widespread viruses ever.
4. Melissa - (1999) The Melissa virus is notable because it is a Word macro virus. It cleverly spread via e-mails sent to contacts from the infected users' address books.
5. Sasser - (2004) Sasser exploited a buffer overflow and spread by connecting to port 445 on networked Windows systems. The chaos caused was possibly the worst ever, as systems restarted or crashed.
6. The Morris Internet Worm - (1988) The grandfather of computer worms, the Morris worm infected Unix systems and was notable for its "accidental" virulence.
7. Blaster - (2003) Blaster exploited a Windows operating system vulnerability and let users know of its presence with a system shutdown warning.
8. SQL Slammer - (2003) This tiny virus infected servers running Microsoft's SQL Server Desktop Engine, and was very fast to spread.
9. Elk Cloner - (1982) Despite Apple's marketing that their systems are less prone to viruses that was not always the case. Notable as possible the first personal computer virus, Elk Cloner infected the boot sector of Apple II floppies.
10. Creeper - (1971) This is noted as possibly the first ever computer virus. It infected computers on ARPANET. Mostly harmless, the concept of Creeper has infected the minds of rogue programmers through today.
Did I miss any you think should have made the list?