Heath Ledger in The Dark Knight (2008)

Heath Ledger's Joker obliterated every previous interpretation of the character with a performance of terrifying, anarchic intensity that remains unmatched by any comic book villain portrayal since. His posthumous Oscar was not a sentimental gesture but recognition of a genuinely revolutionary turn, one that cost him over $1 million in insurance fees due to the extreme nature of his preparation. The performance required 3 months of dedicated rehearsal, and his improvisations added 15% more screen time than the original script allowed.