I haven't had a virus in years but from what I remember, the worse ones were those that slowly start deleting information. I'll never understand what the attacker gets out of that other than being an ass.
I think my biggest hate of viruses is the fake virus program alert. I have many computer illiterate people around me and they always feel the need to do whatever their computer tells them.
If you know your way around the internet, it becomes a challenge to get a virus. If you asked me to name one way to get a virus, I would be at a loss. My best guess would be to start going 4chan and open all the short url links that bots post.