Jared Leto's Joker was born in a storm with Suicide Squad, abused by over-zealous editing and under-served by a story that made him a passenger. By rights, he should have been a footnote in David Ayer's divisive badguy team-up movie, but he's still pretty much the single most definitive element of it.
That reaction, which was as visceral as it was difficult to love says an awful lot about Leto's performance, which was one of the most "complete" performances of any comic book movie. It might not have been entirely successful in everyone's eyes, but the "deep dive" that he took - which he likened to "giving birth out of my prick hole" - led to a stunning, affecting transformation.
And there's a lot more subtlety in the performance than you might give it credit for...
Honourable Mention: His "Unpredictable" Prep
By now, the story of Leto's on-set antics has passed all the way into infamy. It was almost certainly a marketing ploy, given how coordinated the "campaign" by set sources was to leak details of what he'd sent to his fellow cast members.
Allegedly, Leto sent dead animals, used condoms and other delightful Joker-inspired "gifts" and refused to drop character while on set. Leto himself says it was exaggerated, but it's clear that he did go full method and his decision not to rehearse with any of the cast or interact with them around filming WAS designed to assist his character.
He wanted to be unexpected and chaotic and to inspire real, visceral reactions and that's very much a part of what actually works with his Joker.