Casey Chan on Gizmodo has made a good analysis
Have you seen Suicide Squad? Do you like Jared Leto’s Joker or your preferred villain still remains Heath Ledger?
Heath Ledger’s Joker is exceptionally good at attacking Batman’s weakness, rendering Batman’s strength and intimidation useless and using Batman’s moral code (that he can’t kill) against him (because Joker needs to be killed). The Joker also forces Batman to make choices to reveal his true character but also outsmarts him, like when Batman chose to save Rachel over Harvey Dent and ended up with Dent anyway.
But the most important thing that makes Joker the ultimate antagonist is that he and Batman want the same thing: Gotham. Batman wants to save it. The Joker wants it to be in chaos.
And here is a good video made by Lessons from the Screenplay: