The main reason is that this character has a very complex but pure personality. To put it more complexly, anarchism advocates disorder as order, does not believe in human nature, and is good at exploiting the evils of human nature. What is pure is that he is pure evil, and his ideal is crime itself, not profit from crime. This is what Afu said, some people just want to see the world burn. And he and Batman are like two sides of one body on a deeper level. They are like two faces of a complex personality. They have both seen the dirtiest things, and both have clearly understood the ugliness of human nature. But one has played with the world since then. One strives to make people believe in the power of light and is willing to sacrifice everything. The Joker never wanted to physically destroy Batman. He admired this man who had the purest ideas like himself. He wanted to destroy the man's spiritual sustenance and make him become a person like himself.

And Heath Ledger perfectly presented these very difficult-to-act things. I personally like it very much.

I also have a personal opinion. In fact, the old man played by Bale is also the best Wayne in my mind. Only Bruce, who "I am whoever Gotham needs me to be," is worthy of the "why so serious" clown.