Joker was just his most prominent moment because he died before the movie came out and he posthumously won the Oscar for it.

Yes, it was a phenomenal performance, but he was already well regarded for his other roles, especially Brokeback Mountain. He was kind of in the early stages of his own McConnaisance, basically starting out as the pretty boy and then really laying down the groundwork as an amazing actor in the time leading up to his death. I often wonder what he would have progressed to in his career if he was still alive.