I honestly don't know, but I would say CodeRed, 'cause it affected more servers than the others (as far as I know). If a server goes down, you have more problems than if simply one personal computer goes down.
code red, cause its still out there and spreeding, and for people that have netbios open and iis installed they are probably already got code red. Code Red will never go way.
Depends. It would probably be one of the popularized viruses if you're talking about actual, widespread damage. If you're asking about the virus that I'd least like to have, it would be whatever my a friend of mine got. A 'static bomb' of some sort; fried his computer it did.