I think the emnity of Batman and the Joker, as a battle between order / chaos, tragedy driving heroism and mirth driving evil, messianic figure of truth versus Satanic liar and jokester, etc. is about as epic in a mythological sense as you can get. Nolan played off of a lot the available imagery / archetypes associated with those two characters and that's why the story was so rich and multi-layered. I think that's pretty epic. TDK was a crime thriller until the Joker fully enters the scene. Then it picks up into something else... an epic battle of philosophies, with Harvey Dent torn apart in the middle.