According to the production crew, Bane's scar was the result of the poor treatment by the doctor on his spine injury (since supposedly his injuries extended beyond his face). Apparently it's the reason why he wears the large brace around his waist, sort of as a body crutch keeping his spinal muscles in proper alignment (or at least something like that). Another trivia they mentioned was how Bane's intelligence extended beyond tactical planning, as even his military vest was crafted by him by and him alone. It's such a shame that details like those were left out from the movie, as I felt that it really would've enhanced Bane's presence in the film. Don't even get me started on the deleted scenes that elaborated on his backstory.
I wouldn't rule out the Joker just yet. Not so much in disagreement with you, but I thought I'd add to the discussion:
Bane and Joker are different threats. Joker's goal was based on him proving a moralistic point. He wasn't just a loose cannon, and that's part of the character's deception, in that he tried to fool the people that he wasn't at fault due to his "craziness". He wanted to watch the whole world burn as people lost themselves, and that was the point he was trying to prove. It's definitely something that's frightening and because Joker's goal is more of an idea, his tactics tended to be more theatrical and psychological in their execution. Nolan had described Ra's Al Ghul as more of the antagonist with religious ideals, whereas Joker is more of the anti-system foe.
Now on the other hand, Hardy once described Bane's relationship to the criminals in that "some people want to watch the world burn, well Bane is here to pull the pin on the grenade". Unlike Joker, his goals were more absolute. Admittedly this makes his schemes more direct and less theatrical, as he tends to go for the direct source. He took over Gotham under the false pretense of liberation when in fact he established a dictatorship (with plans of killing everyone) for one reason: to torment Bruce Wayne. In contrast to the other two villains, Nolan described Bane's character as more militaristic, in that it combines both ideological and religious notions. He didn't need to prove a point about the city (like Joker), and as such he didn't need any moralistic justification since he recognizes himself as actually being "evil" (unlike Ra's Al Ghul). He wasn't acting like the judge, more than anything he was the sadistic executioner.
Their goals and depiction were most definitely different but both villains were absolutely brilliant in representing what they stood for.
EDIT: Sorry for the walls of text.
it's only because Cillian is my favourite actor, although Hardy isn't far behind.
