It is better than Lobotomy, but Lobotomy isn't particularly good...
It is not a bad card, but I don't see myself using it. It's not all that fast, for one thing. You could easily change that with Dark Ritual. But that same Dark Ritual could power out Hypnotic Specter, Mind Twist, Necropotence, or whatever. Is Cranial Extraction worth the Dark Ritual? Probably not...
Also there is the question of what it's being pointed at. Against control you could name Force of Will or Mana Drain or something. The problem with acting against countermagic in this manner is that if they can counter it, especially with Mana Drain, then they might as well. Duress or Cabal Therapy would get the job done for a single mana, which is A LOT better.
You could also try to name a kill condition or some such card. I can't think of any instances where this card would be particularly good against control, but I could be wrong.
This thing doesn't scratch aggro and I think it would be a wasted play.
It might seem to be a good anti-combo card. This runs into two problems. Firstly, there are already cards that can hose combo decks and do a better job. Secondly, even if it is successful, most combo decks run an alternative kill of some sort, so this doesn't exactly kill the combo player. For example, some TPS builds can Cunning Wish into Brain Freeze or Tinker out Darksteel Colossus. Many Dragon decks run something huge like Sliver Queen which can bust a player up quickly enough.
Prison decks also have multiple ways to kill you and I don't think they are fragile enough for this to be amazing against them.
As for being the best of the COK, I'm not sure. It not exactly an overpowered set. I vote Orchard though, for making Oath all evil and psychotic-like...