There have been many, although the term "bust" is slightly subjective.
Probably the most famous defensive "bust" was Brian Bosworth.
Dan Wilkinson was a consensus first round pick, and although he had a reasonable career, was never the star everyone assumed he would be. Does that make him a bust?
Barry Sanders
1972 Joe Greene, Pittsburgh Steelers Defensive tackle
1974 Joe Greene, Pittsburgh Steelers Defensive tackle
Lawrence Taylor is widely regarded as the most dominant defensive player in history.
Ed Reed of the Baltimore Ravens with 9.
The NFL Defensive Player of the Year for 2010 is Troy Polamalu.
Shawne Merriman
Barry Sanders
Pittsburgh Steelers linebacker James Harrison won the 2008 NFL Defensive Player of the Year Award.
Pittsburgh Steelers strong safety Troy Polamalu won the 2010 NFL Defensive Player of the Year Award.
Danny Cook
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That was defensive tackle Alan Page of the Minnesota Vikings in 1971.
1972 Joe Greene, Pittsburgh Steelers Defensive tackle
1983 Doug Betters, Miami Dolphins Defensive end
1987 Reggie White, Philadelphia Eagles Defensive end
1990 Bruce Smith, Buffalo Bills Defensive end