While he was working on The Imaginarium of Doctor Parnassus at the time of his accidental overdose, there will always be something ultra unsettling about seeing his version of Joker on the big screen.
It also helps that Christopher Nolan’s Batman movies are very memorable and well-regarded by the general population, so the performance has lived on in our collective memories.
This is particularly true given rumors the role allegedly exacerbated some of the sleeping problems he was having in the months leading to his death in 2008.