A mere technicality really but I still feel it's worth mentioning.
A "virus" is a specific thing: "A computer virus is a computer program that can copy itself and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another..." --Wikipedia entry on "computer virus"...always have to take wikipedia with a grain of salt but this actually hits the nail on the head.
I believe you meant "Worst malware ever?"...and I think I'd have to give my nomination to that the stupid adware/malware/fake antivirus crap that screws with the registry so every time you try to run an executable file (.exe...eg: msconfig.exe, regedit.exe, etc), it says "<insert file here> is infected and can not be run. Please buy this crap to fix it" or similar. It gets my vote not because it's particularly destructive but because it's a bit pain in the ass and I'm seeing it become more and more common. I've been running into things like this quite a bit recently.
The most horrible, God awful thing I know of is rootkit malware. I've never run across it thankfully but, from what I've read, some can be VERY difficult to remove. Some will resurface even after a clean OS install...
And, Root...can I change my name to su? :-D