Personally, I don't think Downey, Jr. has ever been better than he was in David Fincher's towering classic, "Zodiac." Here, he plays Paul Avery, a San Francisco Chronicle reporter who becomes unglued after being targeted by the infamous (and, as of today, still unidentified) Zodiac Killer.
The movie is an epic procedural, and Downey Jr. gets to follow the progression of Avery's life in a succinct and brilliant way. It's not gobs of make-up that shows you how he's changed, its subtle changes in his behavior and hair color, the way that he answers questions, how his clothes go from pristine to raggedy. By the time you last see him, sucking down oxygen at a dive bar as he watches his former coworker set out on his own quest, you get the impression that, true to the movie's tagline, there's more than one way to lose your life to a killer.