It doesn't have to be CoD. Here are mine, from #5 to #1:
5: Pandak Panay (Just Cause 2): This little man is seriously evil, not to mention insane. Especially when he has access to four nukes. A great villian, with a spectacular death, too!
4: Imran Zakhaev (COD4): Russian arms dealer so far out of his mind that he's lost the address. Sounds like a charming guy.
3: Frank Tenpenny (GTA SA): A corrupt cop who is only interested in making money, I cheered when this bastard died.
2: General Shepherd (MW2): There are evil people, but it takes a special kind of evil to turn on your own men the way Shepherd did. One cold-hearted SOB.
1: Dimitri Rascalov (GTA IV): Ruthless and disloyal, this Russian mobster is the perfect example of when the American Dream is taken too far.