Sort of depends on what you rank a really bad virus on.
I guess the major characteristics of a bad virus are:
Damage/Payload
Ability to replicate
Stealth/Not be easily detected by AV
Hard to delete
For example, a nuker virus like cadsorin mentioned would rank like this:
Catagories 1,2, and 4 would be 5/5.
It's incredibly powerful, very hard to detect before it strikes, and the fourth category really doesn't apply.
However, it really can't spread, since if it instantly wipes the drive, it cannot replicate to other machines. The only way it would get around is through fake downloads or torrents, and it would hit a hundred computers at the max.
A bad Trojan would really excel in all categories, since it can steal personal information, remain hidden even to good AVs and can be incredibly resilient.