As much as I think Ledger's performance is one of the best acting jobs not just in CBMs, but all movies, there's a case to be made that Nicholson's Joker is the most comic-accurate.
Except he's not.
Not beyond the superficial elements, anyway.
Like, he was certainly a great Joker, but there was no real connection with him and Batman . They happened to wanna plow the same woman and mostly saw each other as nuisances getting in each other's way for the majority of the film, right up until the third act, where all of a sudden it's personal, because Batman needs an excuse to kill him (which he normally doesn't do in comics) because he just then realized he killed his parents (also didn't happen in the comics) and because the film needed to wrap up the story, so in typical movie fashion, the villain must die by the end. Gone is the longevity of Batman and Joker's relationship.
It was definitely accurate in the vat of chemicals falling, the purple suit, the gag weapons etc., But like I said, that's all superficial/surface-level stuff. It fails to really capture the essence of the character and his relationship with Batman, something TDK absolutely NAILED 100% and I still don't see how anyone could say otherwise.
And at least the Phoenix movie does attempt to establish a deeper connection between the two characters who would become Batman and Joker beyond the typical film rivalry in some fashion .
Idk, to each their own, that's just how I've seen it for a while personally.