The new benchmark for modern (solo) superhero movies, The Dark Knight took what director Christopher Nolan teased in Batman Begins and expanded or improved upon it in every appreciable way. That's true of a lot of superhero sequels, but what makes The Dark Knight unique is its improvements don’t stem from bigger set pieces (although it has those) or more effects work. It's simply a better story framed around a unique character (and great performance). From the villain to the scope to the multi-layered plot, The Dark Knight is not just a great Batman film, arguably the best yet, it's a great film in its own right.
Say what you will about the Nolan trilogy as a whole, whether or not the final film delivered, there's no denying The Dark Knight's place in cinema history, as both an excellent comic book adaptation and a huge box office earner.