I really liked him in "A Knights Tale"

To prepare for the role he lived alone in a hotel room for a month, formulating the character's posture, voice and psychology.[8] While he initially found it difficult, Ledger was eventually able to generate a voice which did not sound like Jack Nicholson's take on the character in Tim Burton's 1989 Batman film.[12] He started a diary, in which he wrote the Joker's thoughts and feelings to guide himself during his performance.[9] He was also given Batman: The Killing Joke and Arkham Asylum: A Serious House on Serious Earth to read, which he "really tried to read [...] and put it down".[11] Ledger also cited inspiration in A Clockwork Orange and Sid Vicious.[13] Ultimately, "there’s nothing that [is] consistent", as his main objective was to frighten the audience.[13] The challenge of the role put a strain on Ledger's sleep patterns, finding himself unable to rest for more than a few hours each night.[9]