T2 is an all time classic. However I think The Joker alone > T2 > Dark Knight. The Joker's character just totally transformed the movie to an otherwise good but not outstanding movie.
Heath's acting was just mindblowing, he brought the character to life, to a point where it captivated the audience and even got fans dressing like him, doing the make up, doing impressions and going on an absolute craze. He made it so good in fact that it actually made the concept of evil/villian better than the good guy.