The most horrible virus I know is firmware virus. You can google for it, and there's plenty of info out there.
I've never met one, but a friend of mine working at an internet security firm has encountered some.
If your computer gets infected with one of those, you're fucked. Seriously.
The down side of it is, it's just such a huge waste of time to write one.
Not many crackers out there are actually interested in writing one. You know, in this time-is-money society, crackers would rather infect some newbies' computers and run than sit before desk for weeks just for a virus.
“Artificial Intelligence is no match for Natural Stupidity.”