I'd have to go with Al Qaeda. Their leadership has evaded capture for the past number of decades, despite being hunted by virtually every country in the world. They have paralysed their opponents through terror tactics on numerous occasions. If you counted the sum of the damage they have wrought, it would be billions upon billions. They are currently believed to be present and highly active in Afghanistan, where Coalition forces are currently being defeated (admittedly, that's not due solely to the skill of the opponents, but still).
Don't have to like them, but you have to respect them.