Iron Man.
"Chaplin" and "Less Than Zero" contain his finest performances. As for best, well. He does so much high-class, high-brow, high-quality stuff.
His absloute worst performance and movie was...
Pound(1970) Baby It's You (1983) (uncredited) Firstborn (1984) Deadwait (1985) Tuff Turf (1985) Weird Science (1985) Back to School (1986) America (1986) The Pick-up Artist (1987) Less Than Zero...
Technically, the first movie to star Robert Downey Jr. was an underground film written by his father. It was called Pound and was released in 1970. Downey was only 5 years old.