Loki. This guy is not a good villain. Don't get me wrong, Tom has Loki down through and through and I enjoy his portrayal, but he never feels like a threat - and I don't think he really forces reflection on or from the hero better than other antagonist in the films. My feelings watching Loki usually oscillate between shaking my head at his pettiyness and saying "aahhh, Loki.. not again, you rascal!"
That's precisely the feeling you're supposed to get from Loki. He's the trickster God. He's not taking any of this seriously.