Not really a virus but I managed to create a script which would not only delete the Linux kernel (thankfully only for that one version) it also erased GRUB's config file. Obviously I kinda lost the script because it was a test machine so I just re-installed the OS
Not really a virus because you needed to run it as root in the first place, plus it just sat there, it never moved or anything.
The worst actual virus was on a spare eMachine that I was taking to bits so I first re-installed Windows XP SP1 (no AV or firewall!) and within 2 minutes of getting online the machine had got a virus causing a shutdown before not letting me boot it again. Plus about 10 adwares in that short space of time!
Win!